The author recounts his intellectual development and presents a practical method for attaining certain knowledge, built on rigorous doubt and simple rules for clear and orderly reasoning. He explains how he derived a limited set of methodological precepts for analysis, deduction, and systematic enumeration, then applies them to ethics, metaphysics, and natural philosophy. He offers arguments for the existence of God and for a real distinction between mind and body, sketches mechanistic explanations of physical and biological phenomena, and proposes priorities and institutional aims to accelerate reliable scientific inquiry.