This fundamental quadruple distinction of Entia, which serves as an introduction to the ten Categories or Predicaments, belongs to words altogether according to their relative places or functions in the proposition; the meanings of the words being classified accordingly. That the learner may understand it, he ought properly to be master of the first part of the treatise De Interpretatione, wherein the constituent elements of a proposition are explained: so intimate is the connection between that treatise and this.

The classification applies to Entia (Things or Matters) universally, and is thus a first step in Ontology. He here looks at Ontology in one of its several diverse aspects — as it enters into predication, and furnishes the material for Subjects and Predicates, the constituent members of a proposition.

Ontology, or the Science of Ens quatenus Ens, occupies an important place in Aristotle’s scientific programme; bearing usually the title of First Philosophy, sometimes Theology, though never (in his works) the more modern title of Metaphysica. He describes it as the universal and comprehensive Science, to which all other sciences are related as parts or fractions. Ontology deals with Ens in its widest sense, as an Unum not generic but analogical — distinguishing the derivative varieties into which it may be distributed, and setting out the attributes and accompaniments of Essentia universally; while other sciences, such as Geometry, Astronomy, &c., confine themselves to distinct branches of that whole;11 each having its own separate class of Entia for special and exclusive study. This is the characteristic distinction of Ontology, as Aristotle conceives it; he does not set it in antithesis to Phenomenology, according to the distinction that has become current among modern metaphysicians.

Now Ens (or Entia), in the doctrine of Aristotle, is not a synonymous or univocal word, but an homonymous or equivocal word; or, rather, it is something between the two, being equivocal, with a certain qualification. Though not a Summum Genus, i.e. not manifesting throughout all its particulars generic unity, nor divisible into species by the addition of well-marked essential differentiæ, it is an analogical aggregate, or a Summum Analogon, comprehending under it many subordinates which bear the same name from being all related in some way or other to a common root or fundamentum, the relationship being both diverse in kind and nearer or more distant in degree. The word Ens is thus homonymous, yet in a qualified sense. While it is not univocal, it is at the same time not absolutely equivocal. It is multivocal (if we may coin such a word), having many meanings held together by a multifarious and graduated relationship to one common fundamentum.12 Ens (or Entia), in this widest sense, is the theme of Ontology or First Philosophy, and is looked at by Aristotle in four different principal aspects.13

1. Τὸ ὂν κατὰ συμβεβηκός — Ens per Accidens — Ens accidental, or rather concomitant, either as rare and exceptional attribute to a subject, or along with some other accident in the same common subject.

2. Τὸ ὂν ὡς ἀληθές, καὶ τὸ μὴ ὂν ὡς ψεῦδος — Ens, in the sense /of Truth, Non-Ens, in the sense of Falsehood. This is the Ens of the Proposition; a true affirmation or denial falls under Ens in this mode, when the mental conjunction of terms agrees with reality; a false affirmation or denial, where no such agreement exists, falls under Non-Ens.14

3. Τὸ ὂν δυνάμει καὶ τὸ ὂν ἐνεργείᾳ — Ens, potential, actual.

4. Τὸ ὂν κατὰ τὰ σχήματα τῶν κατηγοριῶν — Ens, according to the ten varieties of the Categories, to be presently explained.

These four are the principal aspects under which Aristotle looks at the aggregate comprised by the equivocal or multivocal word Entia. In all the four branches, the varieties comprised are not species under a common genus, correlating, either as co-ordinate or subordinate, one to the other; they are analoga, all having relationship with a common term, but having no other necessary relationship with each other. Aristotle does not mean that these four modes of distributing this vast aggregate, are the only modes possible; for he himself sometimes alludes to other modes of distributions.15 Nor would he maintain that the four distributions were completely distinguished from each other, so that the same subordinate fractions are not comprehended in any two; for on the contrary, the branches overlap each other and coincide to a great degree, especially the first and fourth. But he considers the four as discriminating certain distinct aspects of Entia or Entitas, more important than any other aspects thereof that could be pointed out, and as affording thus the best basis and commencement for the Science called Ontology.

Of these four heads, however, the first and second are rapidly dismissed by Aristotle in the Metaphysica,16 being conceived as having little reference to real essence, and therefore belonging more to Logic than to Ontology; i.e. to the subjective processes of naming, predicating, believing, and inferring rather than to the objective world of Perceivables and Cogitables.17 It is the third and fourth that are treated in the Metaphysica; while it is the fourth only (Ens according to the ten figures of the Categories) which is set forth and elucidated in this first treatise of the Organon, where Aristotle appears to blend Logic and Ontology into one.

Of this mixed character, partly logical, partly ontological, is the first distinction set forth in the Categoriæ — the distinction between matters predicated of a Subject, and matters which are in a Subject — the Subject itself being assumed as the fundamentum correlative to both of them. The definition given of that which is in a Subject is ontological: viz., “In a Subject, I call that which is in anything, not as a part, yet so that it cannot exist separately from that in which it is.”18 By these two negative characteristics, without any mark positive, does Aristotle define what is meant by being in a Subject. Modern logicians, and Hobbes among them, can find no better definition for an Accident; though Hobbes remarks truly, that Accident cannot be properly defined, but must be elucidated by examples.19

The distinction here drawn by Aristotle between being predicated of a Subject, and being in a Subject, coincides with that between essential and non-essential predication: all the predicates (including the differentia) which belong to the essence, fall under the first division;20 all those which do not belong to the essence, under the latter. The Subjects — what Aristotle calls the First Essences or Substances, those which are essences or substances in the fullest and strictest meaning of the word — are concrete individual things or persons; such as Sokrates, this man, that horse or tree. These are never employed as predicates at all (except by a distorted and unnatural structure of the proposition, which Aristotle indicates as possible, but declines to take into account); they are always Subjects of different predicates, and are, in the last analysis, the Subjects of all predicates. But besides these First Essences, there are also Second Essences — Species and Genus, which stand to the first Essence in the relation of predicates to a Subject, and to the other Categories in the relation of Subjects to predicates.21 These Second Essences are less of Essences than the First, which alone is an Essence in the fullest and most appropriate sense. Among the Second Essences, Species is more of an Essence than Genus, because it belongs more closely and specially to the First Essence; while Genus is farther removed from it. Aristotle thus recognizes a graduation of more or less in Essence; the individual is more Essence, or more complete as an Essence, than the Species, the Species more than the Genus. As he recognizes a First Essence, i.e. an individual object (such as Sokrates, this horse, &c.), so he also recognizes an individual accident (this particular white colour, that particular grammatical knowledge) which is in a Subject, but is not predicated of a Subject; this particular white colour exists in some given body, but is not predicable of any body.22

Respecting the logical distinction, which Aristotle places in the commencement of this treatise on the Categories — between predicates which are affirmed of a Subject, and predicates which are in a Subject23 — we may remark that it turns altogether upon the name by which you describe the predicate. Thus he tells us that the Species and Genus (man, animal), and the Differentia (rational), may be predicated of Sokrates, but are not in Sokrates; while knowledge is in Sokrates, but cannot be predicated of Sokrates; and may be predicated of grammar, but is not in grammar. But if we look at this comparison, we shall see that in the last-mentioned example, the predicate is described by an abstract word (knowledge); while in the preceding examples it is described by a concrete word (man, animal, rational).24 If, in place of these three last words, we substitute the abstract words corresponding to them — humanity, animality, rationality — we shall have to say that these are in Sokrates, though they cannot (in their abstract form) be predicated of Sokrates, but only in the form of their concrete paronyms, which Aristotle treats as a distinct predication. So if, instead of the abstract word knowledge, we employ the concrete word knowing or wise, we can no longer say that this is in Sokrates, and that it may be predicated of grammar. Abstract alone can be predicated of abstract; concrete alone can be predicated of concrete; if we describe the relation between Abstract and Concrete, we must say, The Abstract is in the Concrete — the Concrete contains or embodies the Abstract. Indeed we find Aristotle referring the same predicate, when described by the abstract name, to one Category; and when described by the concrete paronymous adjective, to another and different Category.25 The names Concrete and Abstract were not in the philosophical vocabulary of his day. In this passage of the Categoriæ, he establishes a distinction between predicates essential and predicates non-essential; the latter he here declares to be in the Subject, the former not to be in it, but to be co-efficients of its essence. But we shall find that he does not adhere to this distinction even throughout the present treatise, still less in other works. It seems to be a point of difference between the Categoriæ on one side, and the Physica and Metaphysica on the other, that in the Categoriæ he is more disposed to found supposed real distinctions on verbal etiquette, and on precise adherence to the syntactical structure of a proposition.26

Lastly, Aristotle here makes one important observation respecting those predicates which he describes as (not in a Subject but) affirmed or denied of a Subject — i.e. the essential predicates. In these (he says) whatever predicate can be truly affirmed or denied of the predicate, the same can be truly affirmed or denied of the Subject.27 This observation deserves notice, because it is in fact a brief but distinct announcement of his main theory of the Syllogism; which theory he afterwards expands in the Analytica Priora, and traces into its varieties and ramifications.

After such preliminaries, Aristotle proceeds28 to give the enumeration of his Ten Categories or Predicaments; under one or other of which, every subject or predicate, considered as capable of entering into a proposition, must belong: 1. Essence or Substance; such as, man, horse. 2. How much or Quantity; such as, two cubits long, three cubits long. 3. What manner of or Quality; such as, white, erudite. 4. Ad aliquid — To something or Relation; such as, double, half, greater. 5. Where; such as, in the market-place, in the Lykeium. 6. When; such as, yesterday, last year. 7. In what posture; such as, he stands up, he is sitting down. 8. To have; such as, to be shod, to be armed. 9. Activity; such as, he is cutting, he is turning. 10. Passivity; such as, he is being cut, he is being burned.

Ens in its complete state — concrete, individual, determinate — includes an embodiment of all these ten Categories; the First Ens being the Subject of which the rest are predicates. Whatever question be asked respecting any individual Subject, the information given in the answer must fall, according to Aristotle, under one or more of these ten general heads; while the full outfit of the individual will comprise some predicate under each of them. Moreover, each of the ten is a Generalissimum; having more or fewer species contained under it, but not being itself contained under any larger genus (Ens not being a genus). So that Aristotle does not attempt to define or describe any one of the ten; his only way of explaining is by citing two or three illustrative examples of each. Some of the ten are even of wider extent than Summa Genera; thus, Quality cannot be considered as a true genus, comprehending generically all the cases falling under it. It is a Summum Analogon, reaching beyond the comprehension of a genus; an analogous or multivocal name, applied to many cases vaguely and remotely akin to each other.29 And again the same particular predicate may be ranked both under Quality and under Relation; it need not belong exclusively to either one of them.30 Moreover, Good, like Ens or Unum, is common to all the Categories, but is differently represented in each.31

Aristotle comments at considerable length upon the four first of the ten Categories. 1. Essence or Substance. 2. Quantity. 3. Quality. 4. Relation. As to the six last, he says little upon any of them; upon some, nothing at all.

His decuple partition of Entia or Enunciata is founded entirely upon a logical principle. He looks at them in their relation to Propositions; and his ten classes discriminate the relation which they bear to each other as parts or constituent elements of a proposition. Aristotle takes his departure, not from any results of scientific research, but from common speech; and from the dialectic, frequent in his time, which debated about matters of common life and talk, about received and current opinions.32 We may presume him to have studied and compared a variety of current propositions, so as to discover what were the different relations in which Subjects and Predicates did stand or could stand to each other; also the various questions which might be put respecting any given subject, with the answers suitable to be returned.33

Aristotle ranks as his first and fundamental Category Substance or Essence — Οὐσία; the abstract substantive word corresponding to Τὸ ὄν; which last is the vast aggregate, not generically One but only analogically One, destined to be distributed among the ten Categories as Summa Genera. The First Ens or First Essence — that which is Ens in the fullest sense — is the individual concrete person or thing in nature; Sokrates, Bukephalus, this man, that horse, that oak-tree, &c. This First Ens is indispensable as Subject or Substratum for all the other Categories, and even for predication generally. It is a Subject only; it never appears as a predicate of anything else. As Hic Aliquis or Hoc Aliquid, it lies at the bottom (either expressed or implied) of all the work of predication. It is Ens or Essence most of all, par excellence; and is so absolutely indispensable, that if all First Entia were supposed to be removed, neither Second Entia nor any of the other Categories could exist.34

The Species is recognized by Aristotle as a Second Ens or Essence, in which these First Essences reside; it is less (has less completely the character) of Essence than the First, to which it serves as Predicate. The Genus is (strictly speaking) a Third Essence,35 in which both the First and the Second Essence are included; it is farther removed than the Species from the First Essence, and has therefore still less of the character of Essence. It stands as predicate both to the First and to the Second Essence. While the First Essence is more Essence than the Second, and the Second more than the Third, all the varieties of the First Essence are in this respect upon an equal footing with each other. This man, this horse, that tree, &c., are all Essence, equally and alike.36 The First Essence admits of much variety, but does not admit graduation, or degrees of more or less.