Beattie, in the essay on Truth, part ii. chap. 2, has made some unfounded criticisms on the scepticism of Descartes, and endeavours to turn into ridicule his, Cogito; ergo sum. Yet if any one should deny his own, or our existence, I do not see how we could refute him, were he worthy of refutation, but by some such language; and, in fact, it is what Beattie himself says, more paraphrastically, in answering Hume.

He arrives at more certainty. 88. The certainty of an existing Ego easily led him to that of the operations of the mind, called afterwards by Locke ideas of reflection, the believing, doubting, willing, loving, fearing, which he knew by consciousness, and indeed by means of which alone he knew that the Ego existed. He now proceeded a step farther; and reflecting on the simplest truths of arithmetic and geometry, saw that it was as impossible to doubt of them as of the acts of his mind. But as he had before tried to doubt even of these, on the hypothesis that he might be deceived by a superior intelligent power, he resolved to inquire whether such a power existed, and if it did, whether it could be a deceiver. The affirmative of the former, and the negative of the latter question Descartes established by that extremely subtle reasoning so much celebrated in the seventeenth century, but which has less frequently been deemed conclusive in later times. It is at least that which no man, not fitted by long practise for metaphysical researches, will pretend to embrace.

His proof of a Deity. 89. The substance of his argument was this. He found within himself the idea of a perfect Intelligence, eternal, infinite, necessary. This could not come from himself, nor from external things, because both were imperfect, and there could be no more in the effect than there is in the cause. And this idea requiring a cause, it could have none but an actual being, not a possible being, which is undistinguishable from mere non-entity. If, however, this should be denied, he inquires whether he, with this idea of God, could have existed by any other cause, if there were no God. Not, he argues, by himself; for, if he were the author of his own being, he would have given himself every perfection, in a word, would have been God. Not by his parents, for the same might be said of them, and so forth, if we remount to a series of productive beings. Besides this, as much power is required to preserve as to create, and the continuance of existence in the effect implies the continued operation of the cause.

Another proof of it. 90. With this argument, in itself sufficiently refined, Descartes blended another still more distant from common apprehension. Necessary existence is involved in the idea of God. All other beings are conceivable in their essence, as things possible; in God alone his essence and existence are inseparable. Existence is necessary to perfection; hence, a perfect being, or God, cannot be conceived without necessary existence. Though I do not know that I have misrepresented Descartes in this result of his very subtle argument, it is difficult not to treat it as a sophism. And it was always objected by his adversaries, that he inferred the necessity of the thing from the necessity of the idea, which was the very point in question. It seems impossible to vindicate many of his expressions, from which he never receded in the controversy to which his meditations gave rise. But the long habit of repeating in his mind the same series of reasonings gave Descartes, as it will always do, an inward assurance of their certainty, which could not be weakened by any objection. The former argument for the being of God, whether satisfactory or not, is to be distinguished from the present.[231]

[231] “From what is said already of the ignorance we are in of the essence of mind, it is evident that we are not able to know whether any mind be necessarily existent by a necessity à priori founded in its essence, as we have showed time and space to be. Some philosophers think that such a necessity may be demonstrated of God from the nature of perfection. For God being infinitely, that is, absolutely perfect, they say he must needs be necessarily existent; because, say they, necessary existence is one of the greatest of perfections. But I take this to be one of those false and imaginary arguments, that are founded in the abuse of certain terms; and of all others this word, perfection, seems to have suffered most this way. I wish I could clearly understand what these philosophers mean by the word perfection, when they thus say, that necessity of existence is perfection. Does perfection here signify the same thing that it does, when we say that God is infinitely good, omnipotent, omniscient? Surely perfections are properly asserted of the several powers that attend the essences of things, and not of anything else, but in a very unnatural and improper sense. Perfection is a term of relation, and its sense implies a fitness or agreement to some certain end, and most properly to some power in the thing that is denominated perfect. The term, as the etymology of it shows, is taken from the operation of artists. When an artist proposes to himself to make anything that shall be serviceable to a certain effect, his work is called more or less perfect, according as it agrees more or less with the design of the artist. From arts, by a similitude of sense, this word has been introduced into morality, and signifies that quality of an agent by which it is able to act agreeable to the end its actions tend to. The metaphysicians who reduce everything to transcendental considerations, have also translated this term into their science, and use it to signify the agreement that anything has with that idea, which it is required that thing should answer to. This perfection, therefore, belongs to those attributes that constitute the essence of a thing; and that being is properly called the most perfect which has all, the best, and each the completest in its kind of those attributes, which can be united in one essence. Perfection, therefore, belongs to the essence of things, and not properly to their existence; which is not a perfection of anything, no attribute of it, but only the mere constitution of it in rerum natura. Necessary existence, therefore, which is a mode of existence, is not a perfection, it being no attribute of the thing no more than existence is, which is a mode of it. But it may be said, that though necessary existence is not a perfection in itself, yet it is so in its cause, upon account of that attribute of the entity from whence it flows; that that attribute must of all others be the most perfect and most excellent, which necessary existence flows from, it being such as cannot be conceived otherwise than as existing. But what excellency, what perfection is there in all this? Space is necessarily existent on account of extension, which cannot be conceived otherwise than as existing. But what perfection is there in space upon this account, which can in no manner act on anything, which is entirely devoid of all power, wherein I have showed all perfections to consist? Therefore, necessary existence, abstractedly considered, is no perfection; and, therefore, the idea of infinite perfection does not include, and consequently not prove, God to be necessarily existence [sic]. If he be so, it is on account of those attributes of his essence which we have no knowledge of.”

I have made this extract from a very short tract, called Contemplatio Philosophica, by Brook Taylor, which I found in an unpublished memoir of his life printed by the late Sir William Young, in 1793. It bespeaks the clear and acute understanding of this celebrated philosopher, and appears to me an entire refutation of the scholastic argument of Descartes; one more fit for the Anselms and such dealers in words, from whom it came, than for himself.

His deductions from this. 91. From the idea of a perfect being, Descartes immediately deduced the truth of his belief in an external world, and in the inferences of his reason. For to deceive his creatures would be an imperfection in God; but God is perfect. Whatever, therefore, is clearly and distinctly apprehended by our reason, must be true. We have only to be on our guard against our own precipitancy and prejudice, or surrender of our reason to the authority of others. It is not by our understanding, such as God gave it to us, that we are deceived; but the exercise of our free will, a high prerogative of our nature, is often so incautious as to make us not discern truth from falsehood, and affirm or deny, by a voluntary act, that which we do not distinctly apprehend. The properties of quantity, founded on our ideas of extension and number, are distinctly perceived by our minds, and hence the sciences of arithmetic and geometry are certainly true. But when he turns his thoughts to the phenomena of external sensation, Descartes cannot wholly extricate himself from his original concession, the basis of his doubt, that the senses do sometimes deceive us. He endeavours to reconcile this with his own theory, which had built the certainty of all that we clearly hold certain on the perfect veracity of God.

Primary and secondary qualities. 92. It is in this inquiry that he reaches that important distinction between the primary and secondary properties of matter, the latter being modifications of the former, relative only to our apprehension, but not inherent in things, which, without being wholly new, contradicted the Aristotelian theories of the schools;[232] and he remarked that we are never, strictly speaking, deceived by our senses, but by the inferences which we draw from them.

[232] See Stewart’s First Dissertation on the Progress of Philosophy. This writer has justly observed, that many persons conceive colour to be inherent in the object, so that the censure of Reid on Descartes and his followers, as having pretended to discover what no one doubted, is at least unreasonable in this respect. A late writer has gone so far as to say: “Nothing at first can seem a more rational, obvious, and incontrovertible conclusion, than that the colour of a body is an inherent quality, like its weight, hardness, &c; and that to see the object, and to see it of its own colour, when nothing intervenes between our eyes and it, are one and the same thing. Yet this is only a prejudice.” &c. Herschel’s Discourse on Nat. Philos., p. 82. I almost even suspect that the notion of sounds and smells being secondary or merely sensible qualities, is not distinct in all men’s minds. But after we are become familiar with correct ideas, it is not easy to revive prejudices in our imagination. In the same page of Stewart’s Dissertation, he has been led, by dislike of the university of Oxford, to misconceive, in an extraordinary manner, a passage of Addison in the Guardian, which is evidently a sportive ridicule of the Cartesian theory, and is absolutely inapplicable to the Aristotelian.

93. Such is nearly the substance, exclusive of a great variety of more or less episodical theories, of the three metaphysical works of Descartes, the history of the soul’s progress from opinion to doubt, and from doubt to certainty. Few would dispute, at the present day, that he has destroyed too much of his foundations to render his superstructure stable; and to readers averse from metaphysical reflection, he must seem little else than an idle theorist, weaving cobwebs for pastime which common sense sweeps away. It is fair, however, to observe, that no one was more careful than Descartes to guard against any practical scepticism in the affairs of life. He even goes so far as to maintain, that a man having adopted any practical opinion on such grounds as seem probable should pursue it with as much steadiness as if it were founded on demonstration; observing, however, as a general rule, to choose the most moderate opinions among those which he should find current in his own country.[233]

[233] Vol. i., p. 147. Vol. iii., p. 64.

Objections made to his Meditations. 94. The objections adduced against the Meditations are in a series of seven. The first are by a theologian named Caterus, the second by Mersenne, the third by Hobbes, the fourth by Arnauld, the fifth by Gassendi, the sixth by some anonymous writers, the seventh by a Jesuit of the name of Bourdin. To all of these Descartes replied with spirit and acuteness. By far the most important controversy was with Gassendi, whose objections were stated more briefly, and I think with less skill, by Hobbes. It was the first trumpet in the new philosophy of an ancient war between the sensual and ideal schools of psychology. Descartes had revived, and placed in a clearer light, the doctrine of mind, as not absolutely dependent upon the senses, nor of the same nature as their objects. Stewart does not acknowledge him as the first teacher of the soul’s immateriality. “That many of the schoolmen, and that the wisest of the ancient philosophers, when they described the mind as a spirit, or as a spark of celestial fire, employed these expressions, not with any intention to materialize its essence, but merely from want of more unexceptionable language, might be shown with demonstrative evidence, if this were the proper place for entering into the discussion.”[234] But though it cannot be said that Descartes was absolutely the first who maintained the strict immateriality of the soul, it is manifest to any one who has read his correspondence, that the tenet, instead of being general, as we are apt to presume, was by no means in accordance with the common opinion of his age. The fathers, with the exception, perhaps the single one, of Augustin, had taught the corporeity of the thinking substance. Arnauld seems to consider the doctrine of Descartes as almost a novelty in modern times. “What you have written concerning the distinction between the soul and body appears to me very clear, very evident, and quite divine; and as nothing is older than truth, I have had singular pleasure to see that almost the same things have formerly been very perspicuously and agreeably handled by St. Augustin in all his tenth book on the Trinity, but chiefly in the tenth chapter.”[235] But Arnauld himself, in his objections to the Meditations, had put it as at least questionable, whether that which thinks is not something extended, which, besides the usual properties of extended substances, such as mobility and figure, has also this particular virtue and power of thinking.[236] The reply of Descartes removed the difficulty of the illustrious Jansenist, who became an ardent and almost complete disciple of the new philosophy. In a placard against the Cartesian philosophy, printed in 1647, which seems to have come from Revius, professor of theology at Leyden, it is said: “As far as regards the nature of things, nothing seems to hinder but that the soul may be either a substance, or a mode of corporeal substance.”[237] And More, who had carried on a metaphysical correspondence with Descartes, whom he professed to admire, at least at that time, above all philosophers that had ever existed, without exception of his favourite Plato, extols him after his death in a letter to Clerselier, as having best established the foundations of religion. “For the peripatetics,” he says, “pretend that there are certain substantial forms emanating from matter, and so united to it that they cannot subsist without it, to which class these philosophers refer the souls of almost all living beings, even those to which they allow sensation and thought; while the Epicureans, on the other hand, who laugh at substantial forms, ascribe thought to matter itself, so that it is M. Descartes alone of all philosophers, who has at once banished from philosophy all these substantial forms or souls derived from matter, and absolutely divested matter itself of the faculty of feeling and thinking.”[238]

[234] Dissertation, ubi suprà.

[235] Descartes, x. 138.

[236] Id. ii 14.

[237] Vol. x., p. 73.

[238] Vol. x., p. 386. Even More seems to have been perplexed at one time by the difficulty of accounting for the knowledge and sentiment of disembodied souls, and almost inclined to admit their corporeity. “J’aimerois mieux dire avec les Platoniciens, les anciens pères, et presque tous les philosophes, que les âmes humaines, tous les génies tant bons que mauvais, sont corporels, et que par consequent ils ont un sentiment réel, c’est à dire, qui leur vient du corps dont ils sont revetus.” This is in a letter to Descartes, in 1649, which I have not read in Latin (vol. x., p. 249). I do not quite understand whether he meant only that the soul, when separated from the gross body, is invested with a substantial clothing, or that there is what we may call an interior body, a supposed monad, to which the thinking principle is indissolubly united. This is what all materialists mean, who have any clear notions whatever; it is a possible, perhaps a plausible, perhaps even a highly probable, hypothesis, but one which will not prove their theory. The former seems almost an indispensable supposition, if we admit sensibility to phenomena at all in the soul after death; but it is rather, perhaps, a theological than a metaphysical speculation.

Theory of memory and imagination. 95. It must be owned that the firm belief of Descartes in the immateriality of the Ego or thinking principle, was accompanied with what in later times would have been deemed rather too great concessions to the materialists. He held the imagination and the memory to be portions of the brain, wherein the images of our sensations are bodily preserved; and even assigned such a motive force to the imagination, as to produce those involuntary actions which we often perform, and all the movements of brutes. “This explains how all the motions of all animals arise, though we grant them no knowledge of things, but only an imagination entirely corporeal, and how all those operations which do not require the concurrence of reason are produced in us.” But the whole of his notions as to the connexion of the soul and body, and indeed all his physiological theories, of which he was most enamoured, do little credit to the Cartesian philosophy. They are among those portions of his creed which have lain most open to ridicule, and which it would be useless for us to detail. He seems to have expected more advantage to psychology from anatomical researches than in that state of the science, or even probably in any future state of it, anatomy could afford. When asked once where was his library, he replied, showing a calf he was dissecting, This is my library.[239] His treatise on the passions, a subject so important in the philosophy of the human mind, is made up of crude hypotheses, or at best irrelevant observations, on their physical causes and concomitants.

[239] Descartes was very fond of dissection: C’est un exercise où je me suis souvent occupé depuis onze ans, et je crois qu’il n’y a guère de médecins qui y ait regardé de si près que moi. Vol. viii., p. 100., also p. 174 and 180.

Seat of soul in pineal gland. 96. It may be considered as a part of this syncretism, as we may call it, of the material and immaterial hypothesis, that Descartes fixed the seat of the soul in the conarion, or pineal gland, which he selected as the only part of the brain which is not double. By some mutual communication which he did not profess to explain, though later metaphysicians have attempted to do so, the unextended intelligence, thus confined to a certain spot, receives the sensations which are immediately produced through impressions on the substance of the brain. If he did not solve the problem, be it remembered that the problem has never since been solved. It was objected by a nameless correspondent, who signs himself Hyperaspistes, that the soul being incorporeal could not leave by its operations a trace on the brain, which his theory seemed to imply. Descartes answered, in rather a remarkable passage, that as to things purely intellectual, we do not, properly speaking, remember them at all, as they are equally original thoughts every time they present themselves to the mind, except that they are habitually joined as it were, and associated with certain names, which being bodily, make us remember them.[240]

[240] This passage I must give in French, finding it very obscure, and having translated more according to what I guess than literally. Mais pour ce qui est des choses purement intellectuelles, à proprement parler on n’en aucun ressouvenir; et la première fois qu’elles se présentent à l’esprit, on les pense aussi bien que la seconde, si ce n’est peut-être qu’elles ont coûtume d’être jointes et comme attachées a certains noms qui, étant corporels, font que nous nous ressouvenons aussi d’elles. Vol. viii., p. 271.

Gassendi’s attacks on the Meditations. 97. If the orthodox of the age were not yet prepared for a doctrine which seemed so favourable at least to natural religion as the immateriality of the soul, it may be readily supposed, that Gassendi, like Hobbes, had imbibed too much of the Epicurean theory to acquiesce in the spiritualising principles of his adversary. In a sportive style, he addresses him, O anima! and Descartes, replying more angrily, retorts upon him the name O caro! which he frequently repeats. Though we may lament such unhappy efforts at wit in these great men, the names do not ill represent the spiritual and carnal philosophies; the school that produced Leibnitz, Kant, and Stewart, contrasted with that of Hobbes, Condillac, and Cabanis.

Superiority of Descartes. 98. It was a matter of course that the vulnerable passages of the six Meditations would not escape the spear of so skilful an antagonist as Gassendi. But many of his objections appear to be little more than cavils; and upon the whole, Descartes leaves me with the impression of his great superiority in metaphysical acuteness. It was indeed impossible that men should agree, who persisted in using a different definition of the important word, idea; and the same source of interminable controversy has flowed ever since for their disciples. Gassendi adopting the scholastic maxim, “Nothing is in the understanding, which has not been in the sense,” carried it so much farther than those from whom it came that he denied anything to be an idea but what was imagined by the mind. Descartes repeatedly desired both him and Hobbes, whose philosophy was built on the same notion, to remark that he meant by idea, whatever can be conceived by the understanding, though not capable of being represented by the imagination.[241] Thus we imagine a triangle, but we can only conceive a figure of a thousand sides; we know its existence, and can reason about its properties, but we have no image whatever in the mind, by which we can distinguish such a polygon from one of a smaller or greater number of sides. Hobbes, in answer to this, threw out a paradox which he has not, at least in so unlimited a manner, repeated, that by reason, that is, by the process of reasoning, we can infer nothing as to the nature of things, but only as to their names.[242] It is singular that a man conversant at least with the elements of geometry should have fallen into this error. For it does not appear that he meant to speak only of natural substances, as to which his language might seem to be a bad expression of what was afterwards clearly shown by Locke. That the understanding can conceive and reason upon that which the imagination cannot delineate, is evident not only from Descartes’ instance of a polygon, but more strikingly by the whole theory of infinites, which are certainly somewhat more than bare words, whatever assistance words may give us in explaining them to others or to ourselves.[243]

[241] Par le nom d’idée, il veut seulement qu’on entende ici les images des choses matérielles dépeintes en la fantaisie corporelle; et cela étant supposé, il lui est aisé de montrer qu’on ne peut avoir propre et véritable idée de Dieu ni d’un ange; mais j’ai souvent averti, et principalement en celui là même, que je prends le nom d’idée pour tout ce qui est conçu immédiatement par l’esprit; en sorte que, lorsque je veux et que je crains, parce que je conçois en même temps que je veux et que je crains, ce vouloir et cette crainte sont mis par moi en nombre des idées; et je me suis servi de ce mot, parce qu’il étoit déjà communément reçu par les philosophes pour signifier les formes des conceptions. de l’entendement divin, encore que nous ne reconnoissions en Dieu aucune fantaisie ou imagination corporelle, et je n’en savois point de plus propre. Et je pense avoir assez expliqué l’idée de Dieu pour ceux qui veulent concevoir les sens que je donne à mes paroles; mais pour ceux qui s’attachent à les entendre autrement que je ne fais, je ne le pourrais jamais assez. Vol. i., p. 404. This is in answer to Hobbes; the objections of Hobbes, and Descartes’ replies, turn very much on this primary difference between ideas and images, which alone our countrymen could understand, and ideas as intellections, conceptions, νοουμενα, incapable of being imagined, but not less certainly known and reasoned upon. The French is a translation, but made by Clerselier under the eye of Descartes, so that it may be quoted as an original.

[242] Que dirons nous maintenant si peut-être le raisonnement n’est rien autre chose qu’un assemblage et un enchaînement de noms par ce mot est? D’ou il s’ensuivroit que par la raison nous ne concluons rien de tout touchant la nature des choses, mais seulement touchant leurs appellations, c’est à dire que par elle nous voyons simplement si nous assemblons bien ou mal les noms des choses, selon les conventions que nous avons faites à notre fantaisie touchant leurs significations, p. 476. Descartes merely answered:—L’assemblage qui se fait dans le raisonnement n’est pas celui des noms, mais bien celui des choses signifiées par les noms; et je m’étonne que le contraire puisse venir en l’esprit de personne. Descartes treated Hobbes, whom he did not esteem, with less attention than his other correspondents. Hobbes could not understand what have been called ideas of reflection, such as fear, and thought it was nothing more than the idea of the object feared. “For what else is the fear of a lion,” he says, “than the idea of this lion, and the effect which it produces in the heart, which leads us to run away? But this running is not a thought; so that nothing of thought exists in fear but the idea of the object.” Descartes only replied, “it is self-evident that it is not the same thing to see a lion and fear him, that it is to see him only,” p. 483.

[243] I suspect, from what I have since read, that Hobbes had a different, and what seems to me a very erroneous view of infinite, or infinitesimal quantities in geometry. For he answers the old sophism of Zeno, Quicquid dividi potest in partes infinitas est infinitum, in a manner which does not meet the real truth of the case: Dividi posse in partes infinitas nihil aliud est quam dividi posse in partes quotcunque quis velit. Logica sive Computatio, c. 5., p. 38 (edit. 1667).

Stewart’s remarks on Descartes. 99. Dugald Stewart has justly dwelt on the signal service rendered by Descartes to psychological philosophy, by turning the mental vision inward upon itself, and accustoming us to watch the operations of our intellect, which, though employed upon ideas obtained through the senses, are as distinguishable from them as the workman from his work. He has given indeed to Descartes a very proud title, Father of the experimental philosophy of the human mind, as if he were to man what Bacon was to nature.[244] By patient observation of what passed within him, by holding his soul as it were an object in a microscope, which is the only process of a good metaphysician, he became habituated to throw away those integuments of sense which hide us from ourselves. Stewart has censured him for the paradox, as he calls it, that the essence of mind consists in thinking, and that of matter in extension. That the act of thinking is as inseparable from the mind as extension is from matter, cannot indeed be proved; since, as our thoughts are successive, it is not inconceivable that there may be intervals of duration between them; but it can hardly be reckoned a paradox. But whoever should be led by the word essence to suppose that Descartes confounded the percipient thinking substance, the Ego, upon whose bosom, like that of the ocean, the waves of perception are raised by every breeze of sense, with the perception itself, or even, what is scarcely more tenable, with the reflective action, or thought; that he anticipated this strange paradox of Hume in his earliest work, from which he silently withdrew in his Essays, would not only do great injustice to one of the acutest understandings that ever came to the subject, but overlook several clear assertions of the distinction, especially in his answer to Hobbes. “The thought,” he says, “differs from that which thinks, as the mode from the substance.”[245] And Stewart has in his earliest work justly corrected Reid in this point as to the Cartesian doctrine.[246]

[244] Dissertation on Progress of Philosophy. The word experiment must be taken in the sense of observation. Stewart very early took up his admiration for Descartes. “He was the first philosopher who stated in a clear and satisfactory manner the distinction between mind and matter, and who pointed out the proper plan for studying the intellectual philosophy. It is chiefly in consequence of his precise ideas with respect to this distinction, that we may remark in all his metaphysical writings, a perspicuity which is not observable in those of any of his predecessors.” Elem. of Philos. of Human Mind, vol. i. (published in 1792) note A. “When Descartes,” he says in the dissertation before quoted, “established it as a general principle that nothing conceivable by the power of imagination could throw any light on the operations of thought, a principle which I consider as exclusively his own, he laid the foundations of the experimental philosophy of the human mind. That the same truth had been previously perceived more or less distinctly by Bacon and others, appears probable from the general complexion of their speculations; but which of them has expressed it with equal precision, or laid it down as a fundamental maxim in their logic?” The words which I have put in italics seem too vaguely and not very clearly expressed, nor am I aware that they are borne out in their literal sense, by any position of Descartes; nor do I apprehend the allusion to Bacon. But it is certain that Descartes, and still more his disciples Arnauld and Malebranche, take better care to distinguish what can be imagined from what can be conceived or understood, than any of the school of Gassendi in this or other countries. One of the great merits of Descartes as a metaphysical writer, not unconnected with this, is that he is generally careful to avoid figurative language in speaking of mental operations, wherein he has much the advantage over Locke.

[245] Vol. i., p. 470. Arnauld objected, in a letter to Descartes, Comment se peut il faire que la pensée constitue l’essence de l’esprit, puisque l’esprit est une substance, et que la pensée semble n’en être qu’un mode? Descartes replied that thought in general, la pensée, ou la nature que pense, in which he placed the essence of the soul, was very different from such or such particular acts of thinking, vol. vi., p. 153-160.

[246] Philosophy of Human Mind, vol. i., note A. See the Principia, § 63.

Paradoxes of Descartes. 100. Several singular positions which have led to an undue depreciation of Descartes in general as a philosopher, occur in his metaphysical writings. Such was his denial of thought, and, as is commonly said, sensation to brutes, which he seems to have founded on the mechanism of the bodily organs, a cause sufficient, in his opinion, to explain all the phænomena of the motions of animals, and to obviate the difficulty of assigning to them immaterial souls;[247] his rejection of final causes in the explanation of nature, as far above our comprehension, and unnecessary to those who had the internal proof of God’s existence; his still more paradoxical tenet that the truth of geometrical theorems, and every other axiom of intuitive certainty, depended upon the will of God; a notion that seems to be a relic of his original scepticism, but which he pertinaciously defends throughout his letters.[248] From remarkable errors, men of original and independent genius are rarely exempt; Descartes had pulled down an edifice constructed by the labours of near two thousand years, with great reason in many respects, yet perhaps with too unlimited a disregard of his predecessors; it was his destiny, as it had been theirs, to be sometimes refuted and depreciated in his turn. But the single fact of his having first established, both in philosophical and popular belief, the immateriality of the soul, were we even to forget the other great accessions which he made to psychology, would declare the influence he has had on human opinion. From this immateriality, however, he did not derive the tenet of its immortality. He was justly contented to say that from the intrinsic difference between mind and body, the dissolution of the one could not necessarily take away the existence of the other, but that it was for God to determine whether it should continue to exist; and this determination, as he thought, could only be learned from his revealed will. The more powerful arguments, according to general apprehension, which reason affords for the sentient being of the soul after death, did not belong to the metaphysical philosophy of Descartes, and would never have been very satisfactory to his mind. He says, in one of his letters, that “laying aside what faith assures us of, he owns that it is more easy to make conjectures for our own advantage and entertain promising hopes, than to feel any confidence in their accomplishment.”[249]

[247] It is a common opinion that Descartes denied all life and sensibility to brutes. But this seems not so clear. Il faut remarquer, he says in a letter to More, where he has been arguing against the existence in brutes of any thinking principle, que je parle de la pensée, non de la vie, ou du sentiment; car je n’ôte la vie à aucun animal, ne la faisant consister que dans la seule chaleur du cœur. Je ne leur refuse pas même le sentiment autant qu’il dépend des organes du corps, vol. x., p. 208. In a longer passage, if he does not express himself very clearly, he admits passions in brutes, and it seems impossible that he could have ascribed passions to what has no sensation. Much of what he here says is very good. Bien que Montaigne et Charron aient dit, qu’il y a plus de différence d’homme à homme que d’homme à bête, il n’est toutefois jamais trouvé aucune bête si parfaite, qu’elle ait usé de quelque signe pour faire entendre à d’autres animaux quelque chose que n’eût point de rapport à ses passions; et il n’y a point d’homme si imparfait qu’il n’en use; en sorte que ceux qui sont sourds et muets inventent des signes particuliers par lesquels ils expriment leur pensées; ce qui me semble un très fort argument pour prouver que ce qui fait que les bêtes ne parlent point comme nous, est qu’elles n’ont aucune pensée, et non point que les organes leur manquent. Et on ne peut dire qu’elles parlent entre elles, mais que nous ne les entendons pas; car comme les chiens et quelques autres animaux nous expriment leurs passions, ils nous exprimeroient aussi bien leurs pensées s’ils en avoient. Je sais bien que les bêtes font beaucoup de choses mieux que nous, mais je ne m’en étonne pas; car cela même sert à prouver qu’elles agissent naturellement, et par ressorts, ainsi qu’un horloge; laquelle montre bien mieux l’heure qu’il est, que notre jugement nous l’enseigne.... On peut seulement dire que, bien que les bêtes ne fassent aucune action qui nous assure qu’elles pensent, toutefois, à cause que les organes de leurs corps ne sont pas fort differens des nôtres, on peut conjecturer qu’il y a quelque pensée jointe à ces organes, ainsi que nous experimentons en nous, bien que la leur soit beaucoup moins parfaite; à quoi je n’ai rien à répondre, si non que si elles pensoient aussi que nous, elles auroient une ame immortelle aussi bien que nous; ce qui n’est pas vraisemblable, à cause qu’il n’y a point de raison pour le croire de quelques animaux, sans le croire de tous, et qu’il y en a plusieurs trop imparfaits pour pouvoir croire cela d’eux, comme sont les huitres, les éponges, &c. Vol. ix., p. 425. I do not see the meaning of une ame immortelle in the last sentence; if the words had been une ame immortelle, it would be to the purpose. More, in a letter to which this is a reply, had argued as if Descartes took brutes for insensible machines, and combats the paradox with the arguments which common sense furnishes. He would even have preferred ascribing immortality to them, as many ancient philosophers did. But surely Descartes, who did not acknowledge any proofs of the immortality of the human soul to be valid, except those founded on revelation, needed not to trouble himself much about this difficulty.

[248] C’est en effet parler de Dieu comme d’un Jupiter ou d’un Saturne, et l’assujettir au Styx et aux destinées, que de dire que ces vérités sont indépendantes de lui. Ne craignez point, je vous prie, d’assurer et de publier partout que c’est Dieu qui a établi ces lois en la nature, ainsi qu’un roi établit les lois en son royaume. Vol. vi., p. 109. He argues as strenuously the same point in p. 132 and p. 307.

[249] Vol. ix., p. 369.

His just notion of definitions. 101. Descartes was perhaps the first who saw that definitions of words, already as clear as they can be made, are nugatory or impenetrable. This alone would distinguish his philosophy from that of the Aristotelians, who had wearied and confused themselves for twenty centuries with unintelligible endeavours to grasp by definition what refuses to be defined. “Mr Locke,” says Stewart, “claims this improvement as entirely his own, but the merit of it unquestionably belongs to Descartes, although it must be owned that he has not always sufficiently attended to it in his researches.”[250] A still more decisive passage to this effect, than that referred to by Stewart in the Principia will be found in the posthumous dialogue on the Search after Truth. It is objected by one of the interlocutors, as it had actually been by Gassendi, that, to prove his existence by the act of thinking, he should first know what existence and what thought is. “I agree with you,” the representative of Descartes replies, “that it is necessary to know what doubt is, and what thought is, before we can be fully persuaded of this reasoning; I doubt, therefore I am, or what is the same, I think, therefore I am. But do not imagine that for this purpose you must torture your mind to find out the next genus, or the essential differences, as the logicians talk, and so compose a regular definition. Leave this to such as teach or dispute in the schools. But whoever will examine things by himself, and judge of them according to his understanding, cannot be so senseless as not to see clearly, when he pays attention, what doubting, thinking, being, are, and as to have any need to learn their distinctions. Besides, there are things which we render more obscure, in attempting to define them, because, as they are very simple and very clear, we cannot know and comprehend them better than by themselves. And it should be reckoned among the chief errors that can be committed in science for men to fancy that they can define that which they can only conceive, and distinguish what is clear in it from what is obscure, while they do not see the difference between that which must be defined before it is understood and that which can be fully known by itself. Now, among things which can thus be clearly known by themselves, we must put doubting, thinking, being. For I do not believe any one ever existed so stupid as to need to know what being is before he could affirm that he is; and it is the same of thought and doubt. Nor can he learn these things except by himself, nor be convinced of them but by his own experience, and by that consciousness and inward witness which every man finds in himself when he examines the subject. And as we should define whiteness in vain to a man who can see nothing, while one can open his eyes and see a white object requires no more, so to know what doubting is, and what thinking is, it is only necessary to doubt and to think.”[251] Nothing could more tend to cut short the verbal cavils of the schoolmen, than this limitation of their favourite exercise, definition. It is due, therefore, to Descartes, so often accused of appropriating the discoveries of others, that we should establish his right to one of the most important that the new logic has to boast.

[250] Dissertation, ubi, suprá, Stewart, in his Philosophical Essays, note A, had censured Reid for assigning this remark to Descartes and Locke, but without giving any better reason than that it is found in a work written by Lord Stair; earlier, certainly, than Locke, but not before Descartes. It may be doubtful, as we shall see hereafter, whether Locke has not gone beyond Descartes, or at least distinguished undefinable words more strictly.

[251] Vol. xi., p. 369.

His notion of substances. 102. He seems, at one moment, to have been on the point of taking another step very far in advance of his age. “Let us take,” he says, “a piece of wax from the honey-comb; it retains some taste and smell, it is hard, it is cold, it has a very marked colour, form, and size. Approach it to the fire; it becomes liquid, warm, inodorous, tasteless; its form and colour are changed, its size is increased. Does the same wax remain after these changes? It must be allowed that it does; no one doubts it, no one thinks otherwise. What was it then that we so distinctly knew to exist in this piece of wax? Nothing certainly that we observed by the senses, since all that the taste, the smell, the sight, the touch reported to us has disappeared, and still the same wax remains.” This something which endures under every change of sensible qualities cannot be imagined; for the imagination must represent some of these qualities, and none of them are essential to the thing; it can only be conceived by the understanding.[252]

[252] Méditation Seconde, i. 256.

Not quite correct. 103. It may seem almost surprising to us, after the writings of Locke and his followers on the one hand, and the chemist with his crucible on the other, have chased these abstract substances of material objects from their sanctuaries, that a man of such prodigious acuteness and intense reflection as Descartes should not have remarked that the identity of wax after its liquefaction is merely nominal, and depending on arbitrary language, which in many cases gives new appellations to the same aggregation of particles after a change of their sensible qualities; and that all we call substances are but aggregates of resisting moveable corpuscles, which by the laws of nature are capable of affecting our senses differently, according to the combinations they may enter into, and the changes they may successively undergo. But if he had distinctly seen this, which I do not apprehend that he did, it is not likely that he would have divulged the discovery. He had already given alarm to the jealous spirit of orthodoxy by what now appears to many so self-evident, that they have treated the supposed paradox as a trifling with words, the doctrine that colour, heat, smell, and other secondary qualities, or accidents of bodies, do not exist in them, but in our own minds, and are the effects of their intrinsic or primary qualities. It was the the tenet of the schools that these were sensible realities, inherent in bodies; and the church held as an article of faith, that the substance of bread being withdrawn from the consecrated wafer, the accidents of that substance remained as before, but independent, and not inherent in any other. Arnauld raised this objection, which Descartes endeavoured to repel by a new theory of transubstantiation; but it always left a shade of suspicion, in the Catholic church of Rome, on the orthodoxy of Cartesianism.

His notions of intuitive truth. 104. “The paramount and indisputable authority which, in all our reasonings concerning the human mind, he ascribes to the evidence of consciousness” is reckoned by Stewart among the great merits of Descartes. It is certain that there are truths which we know, as it is called, intuitively, that is, by the mind’s immediate inward glance. And reasoning would be interminable, if it did not find its ultimate limit in truths which it cannot prove. Gassendi imputed to Descartes, that, in his fundamental enthymem, Cogito, ergo sum, he supposed a knowledge of the major premise, Quod cogitat, est. But Descartes replied that it was a great error to believe that our knowledge of particular propositions must always be deduced from universals, according to the rules of logic; whereas, on the contrary, it is by means of our knowledge of particulars that we ascend to generals, though it is true that we descend again from them to infer other particular propositions.[253] It is probable that Gassendi did not make this objection very seriously.

[253] Vol. ii., p. 305. See too the passage, quoted above, in his posthumous dialogue.

105. Thus the logic of Descartes, using that word for principles that guide our reasoning, was an instrument of defence both against the captiousness of ordinary scepticism, that of the Pyrrhonic school, and against the disputatious dogmatism of those who professed to serve under the banner of Aristotle. He who reposes on his own consciousness, or who recurs to first principles of intuitive knowledge, though he cannot be said to silence his adversary, should have the good sense to be silent himself, which puts equally an end to debate. But so far as we are concerned with the investigation of truth, the Cartesian appeal to our own consciousness, of which Stewart was very fond, just as it is in principle, may end in an assumption of our own prejudices as the standard of belief. Nothing can be truly self-evident, but that which a clear, an honest, and an experienced understanding in another man acknowledges to be so.

Treatise on art of logic. 106. Descartes has left a treatise highly valuable, but not very much known, on the art of logic, or rules for the conduct of the understanding.[254] Once only, in a letter, he has alluded to the name of Bacon.[255] There are perhaps a few passages in this short tract that remind us of the Novum Organum. But I do not know that the coincidence is such as to warrant a suspicion that he was indebted to it; we may reckon it rather a parallel, than a derivative logic; written in the same spirit of cautious, inductive procedure, less brilliant and original in its inventions, but of more general application than the Novum Organum, which is with some difficulty extended beyond the province of natural philosophy. Descartes is as averse as Bacon to syllogistic forms. “Truth,” he says, “often escapes from these fetters, in which those who employ them remain entangled. This is less frequently the case with those who make no use of logic, experience showing that the most subtle of sophisms cheat none but sophists themselves, not those who trust to their natural reason. And to convince ourselves how little this syllogistic art serves towards the discovery of truth, we may remark that the logicians can form no syllogism with a true conclusion, unless they are already acquainted with the truth that the syllogism develops. Hence it follows that the vulgar logic is wholly useless to him who would discover truth for himself, though it may assist in explaining to others the truth he already knows, and that it would be better to transfer it as a science from philosophy to rhetoric.”[256]

[254] M. Cousin has translated and republished two works of Descartes, which had only appeared in Opera Posthuma Cartesii, Amsterdam, 1701. Their authenticity, from external and intrinsic proofs, is out of question. One of these is that mentioned in the text; entitled “Rules for the Direction of the Understanding;” which, though logical in its subject, takes most of its illustrations from mathematics. The other is a dialogue, left imperfect, in which he sustains the metaphysical principles of his philosophy. Of these two little tracts, their editor has said, that “they equal in vigour and perhaps surpass in arrangement the Meditations and Discourse on Method. We see in these more unequivocally the main object of Descartes, and the spirit of the revolution which has created modern philosophy, and placed in the understanding itself the principle of all certainty, the point of departure for all legitimate inquiry. They might seem written but yesterday, and for the present age.” Vol. xi. preface, p. 1. I may add to this, that I consider the Rules for the direction of the Understanding as one of the best works on logic (in the enlarged sense) which I have ever read; more practically useful, perhaps, to young students than the Novum Organum; and though, as I have said, his illustrations are chiefly mathematical, most of his rules are applicable to the general discipline of the reasoning powers. It occupies little more than one hundred pages, and I think that I am doing a service in recommending it. Many of the rules will, of course, be found in later books; some possibly in earlier. This tract, as well as the dialogue which follows it, is incomplete, a portion being probably lost.

[255] Si quelqu’un de cette humeur vouloit entreprendre d’écrire l’histoire des apparences célestes selon la méthode de Verulamius. Vol. vi., p. 210.

[256] Vol. xi., p. 255.

Merits of his writings. 107. It would occupy too much space to point out the many profound and striking thoughts which this treatise on the conduct of the understanding, and indeed most of the writings of Descartes contain. “The greater part of the questions on which the learned dispute are but questions of words. These occur so frequently that, if philosophers would agree on the signification of their words, scarce any of their controversies would remain.” This has been continually said since; but it is a proof of some progress in wisdom, when the original thought of one age becomes the truism of the next. No one had been so much on his guard against the equivocation of words, or knew so well their relation to the operations of the mind. And it may be said, generally, though not without exception of the metaphysical writings of Descartes, that we find in them a perspicuity which springs from his unremitting attention to the logical process of inquiry, admitting no doubtful or ambiguous position, and never requiring from his reader a deference to any authority but that of demonstration. It is a great advantage in reading such writers that we are able to discern when they are manifestly in the wrong. The sophisms of Plato, of Aristotle, of the schoolmen, and of a great many recent metaphysicians, are disguised by their obscurity; and while they creep insidiously into the mind of the reader, are always denied and explained away by partial disciples.

His notions of free will. 108. Stewart has praised Descartes for having recourse to the evidence of consciousness in order to prove the liberty of the will. But he omits to tell us that the notions entertained by this philosopher were not such as have been generally thought compatible with free agency in the only sense that admits of controversy. It was an essential part of the theory of Descartes that God is the cause of all human actions. “Before God sent us into the world,” he says in a letter, “he knew exactly what all the inclinations of our will would be; it is he that has implanted them in us; it is he also that has disposed all other things, so that such or such objects should present themselves to us at such or such times, by means of which he has known that our free will would determine us to such or such actions, and he has willed that it should be so; but he has not willed to compel us thereto.”[257] “We could not demonstrate,” he says at another time, “that God exists, except by considering him as a being absolutely perfect; and he could not be absolutely perfect, if there could happen anything in the world which did not spring entirely from him.... Mere philosophy is enough to make us know that there cannot enter the least thought into the mind of man, but God must will and have willed from all eternity that it should enter there.”[258] This is in a letter to his highly intelligent friend, the princess Palatine Elizabeth, granddaughter of James I.; and he proceeds to declare himself strongly in favour of predestination, denying wholly any particular providence, to which she had alluded, as changing the decrees of God, and all efficacy of prayer, except as one link in the chain of his determinations. Descartes, therefore, whatever some of his disciples may have become, was far enough from an Arminian theology. “As to free will,” he says elsewhere, “I own that thinking only of ourselves we cannot but reckon it independent, but when we think of the infinite power of God we cannot but believe that all things depend on him, and that consequently our free will must do so too.... But since our knowledge of the existence of God should not hinder us from being assured of our free will, because we feel and are conscious of it in ourselves, so that if our free will should not make us doubt of the existence of God. For the independence which we experience and feel in ourselves, and which is sufficient to make our actions praiseworthy or blameable, is not incompatible with a dependence of another nature, according to which all things are subject to God.”[259]

[257] Vol. ix., p. 374.

[258] Id. p. 246.

[259] Vol. ix., p. 368. This had originally been stated in the Principia with less confidence, the free will of man and predetermination of God being both asserted as true, but their co-existence incomprehensible. Vol. iii., p. 86.

Fame of his system and attacks upon it. 109. A system so novel, so attractive to the imagination by its bold and brilliant paradoxes as that of Descartes, could not but excite the attention of an age already roused to the desire of a new philosophy, and to the scorn of ancient authority. His first treatises appeared in French; and, though he afterwards employed Latin, his works were very soon translated by his disciples, and under his own care. He wrote in Latin with great perspicuity; in French with liveliness and elegance. His mathematical and optical writings gave him a reputation which envy could not take away, and secured his philosophy from that general ridicule which sometimes overwhelms an obscure author. His very enemies, numerous and vehement as they were, served to enhance the celebrity of the Cartesian system, which he seems to have anticipated by publishing their objections to his Meditations with his own replies. In the universities, bigoted for the most part to Aristotelian authority, he had no chance of public reception; but the influence of the universities was much diminished in France, and a new theory had perhaps better chances in its favour on account of their opposition. But the Jesuits, a more powerful body, were in general adverse to the Cartesian system, and especially some time afterwards, when it was supposed to have the countenance of several leading Jansenists. The Epicurean school, led by Gassendi and Hobbes, presented a formidable phalanx; since it in fact comprehended the wits of the world, the men of indolence and sensuality, quick to discern the many weaknesses of Cartesianism, with no capacity for its excellencies. It is unnecessary to say, how predominant this class was in the seventeenth and eighteenth centuries, both in France and England.

Controversy with Voet. 110. Descartes was evidently in considerable alarm lest the church should bear with its weight upon his philosophy.[260] He had the censure on Galileo before his eyes, and certainly used some chicane of words as to the earth’s movement upon this account. It was, however, in the Protestant country, which he had chosen as his harbour of refuge, that he was doomed to encounter the roughest storm. Gisbert Voet, an eminent theologian in the university of Utrecht, and the head of the party in the church of Holland, which had been victorious in the synod of Dort, attacked Descartes with all the virulence and bigotry characteristic of his school of divinity. The famous demonstration of the being of God he asserted to be a cover for atheism, and thus excited a flame of controversy, Descartes being not without supporters in the university, especially Regius, professor of medicine. The philosopher was induced by these assaults to change his residence from a town in the province of Utrecht to Leyden. Voet did not cease to pursue him with outrageous calumny, and succeeded in obtaining decrees of the senate and university, which interdicted Regius from teaching that “new and unproved (præsumpta) philosophy” to his pupils. The war of libels on the Voetian side did not cease for some years, and Descartes replied with no small acrimony against Voet himself. The latter had recourse to the civil power, and instituted a prosecution against Descartes, which was quashed by the interference of the prince of Orange. But many in the university of Leyden, under the influence of a notable theologian of that age, named Triglandius, one of the stoutest champions of Dutch orthodoxy, raised a cry against the Cartesian philosophy as being favourable to Pelagianism and popery, the worst names that could be given in Holland; and it was again through the protection of the prince of Orange that he escaped a public censure. Regius, the most zealous of his original advocates, began to swerve from the fidelity of a sworn disciple, and published a book containing some theories of his own, which Descartes thought himself obliged to disavow. Ultimately he found, like many benefactors of mankind, that he had purchased reputation at the cost of peace; and, after some visits to France, where, probably from the same cause, he never designed to settle, found an honourable asylum and a premature death at the court of Christina. He died in 1651, having worked a more important change in speculative philosophy than any who had preceded him since the revival of learning; for there could be no comparison, in that age, between the celebrity and effect of his writings and those of Lord Bacon. The latter had few avowed enemies, till it was too late to avow enmity.[261]

[260] On a tellement assujetti la théologie à Aristotle, qu’il est impossible d’expliquer une autre philosophie qu’il ne semble d’abord qu’elle soit contre la foi. Et à propos de ceci, je vous prie de me mander s’il n’y a rien de déterminé en la foi touchant l’étendue du monde: savoir s’il est fini ou plutôt infini, et si tout ce qu’on appelle espaces imaginaires soient des corps créés et véritables. Vol. vi., p. 73.

[261] The life of Descartes was written, very fully and with the warmth of a disciple, by Baillet, in two volumes quarto, 1691, of which he afterwards published an abridgment. In this we find at length the attacks made on him by the Voetian theologians. Brucker has given a long and valuable account of the Cartesian philosophy, but not favourable, and perhaps not quite fair. Vol. v., p. 200-334. Buhle is, as usual, much inferior to Brucker. But those who omit the mathematical portion will not find the original works of Descartes very long, and they are well worthy of being read.

Charges of plagiarism. 111. The prejudice against Descartes, especially in his own country, was aggravated by his indiscreet and not very warrantable assumption of perfect originality.[262] No one, I think, can fairly refuse to own, that the Cartesian metaphysics, taken in their consecutive arrangement, form truly an original system; and it would be equally unjust to deny the splendid discoveries he developed in algebra and optics. But upon every one subject which Descartes treated, he has not escaped the charge of plagiarism; professing always to be ignorant of what had been done by others, he falls perpetually into their track; more, as his adversaries maintained, than the chances of coincidence could fairly explain. Leibnitz has summed up the claims of earlier writers to the pretended discoveries of Descartes; and certainly it is a pretty long bill to be presented to any author. I shall insert this passage in a note, though much of it has no reference to this portion of the Cartesian philosophy.[263] It may perhaps be thought by candid minds, that we cannot apply the doctrine of chances to coincidence of reasoning in men of acute and inquisitive spirits, as fairly as we may to that of style or imagery; but, if we hold strictly that the older writer may claim the exclusive praise of the philosophical discovery, we must regret to see such a multitude of feathers plucked from the wing of an eagle.

[262] I confess, he says in his logic, that I was born with such a temper, that the chief pleasure I find in study is not from the learning the arguments of others, but by inventing my own. This disposition alone impelled me in youth to the study of science; hence, whenever a new book promised by its title some new discovery, before sitting down to read it, I used to try whether my own natural sagacity could lead me to anything of the kind, and I took care not to lose this innocent pleasure by too hasty a perusal. This answered so often that I at length perceived that I arrived at truth, not as other men do after blind and precarious guesses, by good luck rather than skill, but that long experience had taught me certain fixed rules, which were of surprising utility, and of which I afterwards made use to discover more truths. Vol. xi., p. 252.

[263] Dogmata ejus metaphysica, velut circa ideas a sensibus remotas, et animæ distinctionem a corpore, et fluxam per se rerum materialium fidem, prorsus Platonica sunt. Argumentum pro existentia Dei, ex eo, quod ens perfectissimum, vel quo majus intelligi non potest, existentiam includit, fuit Anselmi, et in libro “Contra insipientem” inscripto extat inter ejus opera, passimque a scholasticis examinatur. In doctrina de continuo, pleno et loco Aristotelem noster secutus est, Stoicosque in re morali penitus expressit, floriferis ut apes in saltibus omnia libant. In explicatione rerum mechina Leucippum et Democritum præeuntes habuit, qui et vortices ipsos jam docuerant. Jordanus Brunus easdem fere de magnitudine universi ideas habuisse dicitur, quemadmodum et notavit V. CC. Stephanus Spleissius, ut de Gilberto nil dicam, cujus magneticæ considerationes tum per se, tum ad systema universi applicatæ, Cartesio plurimum profuerunt. Explicationem gravitatis per materiæ solidioris rejectionem in tangente, quod in physica Cartesiana prope pulcherrimum est, didicit ex Keplero, qui similitudine palearum motu aquæ in vase gyrantis ad centrum contrusarum rem explicuit primus. Actionem lucis in distans, similitudine baculi pressi jam veteres adumbravere. Circa iridem a M. Antonio de Dominis non parum lucis accepit. Keplerum fuisse primum suum in dioptricis magistrum, et in eo argumento omnes ante se mortales longo intervallo antegressum, fatetur Cartesius in epistolis familiaribus; nam in scriptis, quæ ipse edidit, longè abest a tali confessione aut laude, tametsi illa ratio, quæ rationum dictionem explicat, et compositione nimirum duplicis conatûs perpendicularis ad superficiem et ad eandem paralleli, disertè apud Keplerum extet, qui eodem, ut Cartesius, modo æqualitatem angulorum incidentiæ et reflexionis hinc deducit. Idque gratam mentionem ideo merebatur, quod omnis prope Cartesii ratiocinatio huic innititur principio. Legem refractionis primum invenisse Willebroodum Snellium, Isaacus Vossius patefecit, quanquam non ideo negare ausim, Cartesium in eadem incidere potuisse de suo. Negavit in epistolis Vietam sibi lectum, sed Thomæ Harrioti Angli libros analyticos posthumos anno 1631 editos vidisse multi vix dubitant; usque adeo magnus est eorum consensus cum calculo geometriæ Cartesianæ. Sane jam Harriotus æquationem nihilo æqualem posuit, et hinc derivavit, quomodo oriatur æquatio ex multiplicatione radicum in se invicem, et quomodo radiorum auctione, diminutione, multiplicatione aut divisione variari æquatio possit, et quomodo proinde natura, et constitutio æquationem et radicum cognosci possit ex terminorum habitudine. Itaque narrat celeberrimus Wallisius, Robervalium, qui miratus erat, unde Cartesio in mentem venisset palmarium illud, æquationem ponere æqualem nihilo ad instar unius quantitatis, ostenso sibi a Domino de Cavendish libro Harrioti exclamasse, il l’a vu! il l’a vu! vidit, vidit. Reductionem quadratoquadratæ æquationis ad cubicam superiori jam sæculo invenit Ludovicus Ferrarius, cujus vitam reliquit Cardanus ejus familiaris. Denique fuit Cartesius, ut a viris doctis dudum notatum est, et ex epistolis nimium apparet, immodicus contemptor aliorum, et famæ cupiditate ab artificiis non abstinens, quæ parum generosa videri possunt. Atque hæc profecto non dico animo obtrectandi viro, quem mirificè æstimo, sed eo consilio, ut cuique suum tribuatur, nec unus omnium laudes absorbeat; justissimum enim est, ut inventoribus suus honos constet, nec sublatis virtutum præmiis præclara faciendi studium refrigescat. Leibnitz, apud Brucker, v. 255.