67. Application to Psychology. -- Knowledge, its Origin and Nature. -- All knowledge is sensation, and this latter owes its origin to atomic emanations (10). The repetition of sensations engenders the concept or general image (prolêpsis) which becomes fixed in the memory. We pass from the known to the unknown by opinion (doxa) which is merely a judgment or reasoning about sensations. Certitude exists: Epicurus proves this like the Stoics by appealing to moral considerations. What is the criterion of certitude? The very existence of the sensation. By the very fact that it exists, a sensation is true, in conformity with its object. For its object is not the exterior thing, but the image produced in us by that exterior thing. In Epicurus' system of criteriology, errors of the senses find no place. Error arises only when by judgment we attribute to the things themselves what is only true of their images in us. This theory leads logically to the subjectivism of Protagoras (12). In practice, however, Epicurus admitted that our perceptions attain not only to the things as represented, but also to the things in themselves. The concept has the same claim to certitude as the sensation. As for opinion, it is true or false according as it is confirmed or not by experience.

The Will is a mechanical movement of the soul; but Epicurus does not attempt to explain it. All his attention is concentrated on the problem of liberty. Just as the Stoics, who held that morality consists in man's submission to the cosmic laws, insisted on psychological determinism, so the Epicureans, who placed happiness in man's individualism and absolute independence, based their belief on free will. It is just in order to safeguard the possibility of a free act in his exaggerated mechanical theory, that Epicurus attributed to the atoms a quasi-voluntary power to deviate from the perpendicular. Logic would oblige him to endow every atom of matter in the universe with liberty, the monopoly of which he reserves so jealously for the human being.

Nature of the Soul. -- The soul is corporeal. The atoms which compose it are the lightest and most mobile: it results from a mixture of fire, air, pneuma, and another element infinitely mobile. It permeates the whole body, but the intellectual part rules supreme. The soul comes into the world with the body; at death it dissolves into the ether: a consoling thought seeing that death is thus the end of all painful sensations!