Beginning with _Canonics_, as the first part of philosophy in order of time, from the point of view of human knowledge, Epicurus laid it down that the only source of knowledge was the senses, which gave us an immediate and true perception of that which actually came into contact with them. Even the visions of madmen or of dreamers he considered were in themselves true, being produced by a physical cause of some kind, of which these visions were the direct and immediate report. Falsity came in with {215} people's interpretations or imaginations with respect to these sensations. Sensations leave a trace in the memory, and out of similarities or analogies among sensations there are developed in the mind general notions or types, such as 'man,' 'house,' which are also true, because [373] they are reproductions of sensations. Thirdly, when a sensation occurs, it is brought into relation in the mind with one or more of these types or notions; this is _predication_, true also in so far as its elements are true, but capable of falsehood, as subsequent or independent sensation may prove. If supported or not contradicted by sensation, it is or may be true; if contradicted or not supported by sensation, it is or may be false. The importance of this statement of the canon of truth or falsehood will be understood when we come to the physics of Epicurus, at the basis of which is his theory of Atoms, which by their very nature can never be directly testified to by sensation. [374] This and no more was what Epicurus had to teach on the subject of logic. He had no theory of definition, or division, or ratiocination, or refutation, or explication; on all these matters Epicurus was, as Cicero said, 'naked and unarmed.' Like most self-taught or ill-taught teachers, Epicurus trusted to his dogmas; he knew nothing and cared nothing for logical defence. {216} [375] In his _Physics_ Epicurus did little more than reproduce the doctrine of Democritus. He starts from the fundamental proposition that 'nothing can be produced from nothing, nothing can really perish.' The veritable existences in nature are the Atoms, which are too minute to be discernible by the senses, but which nevertheless have a definite size, and cannot further be divided. They have also a definite weight and form, but no qualities other than these. There is an infinity of empty space; this Epicurus proves on abstract grounds, practically because a limit to space is unthinkable. It follows that there must be an infinite number of the atoms, otherwise they would disperse throughout the infinite void and disappear. There is a limit, however, to the number of varieties among the atoms in respect of form, size, and weight. The existence of the void space is proved by the fact that motion takes place, to which he adds the argument that it necessarily exists also to separate the atoms one from another. So far Epicurus and Democritus are agreed.