Since all knowledge is a knowledge of sense-objects, truth is simply the correspondence of our impressions to things. How are we to know whether our ideas are correct copies of things? How distinguish between reality and imagination, dreams, or illusions? What is the criterion of truth? It cannot lie in concepts, since these are of our own making. Nothing is true save {346} sense-impressions, and therefore the criterion of truth must lie in sensation itself. It cannot be in thought, but must be in feeling. Real objects, said the Stoics, produce in us an intense feeling, or conviction, of their reality. The strength and vividness of the image distinguish these real perceptions from a dream or fancy. Hence the sole criterion of truth is this striking conviction, whereby the real forces itself upon our consciousness, and will not be denied. The relapse into complete subjectivity will here be noted. There is no universally grounded criterion of truth. It is based, not on reason, but on feeling. All depends on the subjective convictions of the individual. Physics. The fundamental proposition of the Stoic physics is that "nothing incorporeal exists." This materialism coheres with the sensationalism of their doctrine of knowledge. Plato placed knowledge in thought, and reality, therefore, in the Idea. The Stoics, however, place knowledge in physical sensation, and reality, therefore, in what is known by the senses, matter. All things, they said, even the soul, even God himself, are material and nothing more than material. This belief they based upon two main considerations. Firstly, the unity of the world demands it. The world is one, and must issue from one principle. We must have a monism. The idealism of Plato and Aristotle had resolved itself into a futile struggle against the dualism of matter and thought. Since the gulf cannot be bridged from the side of the Idea, we must take our stand on matter, and reduce mind to it. Secondly, body and soul, God and {347} the world, are pairs which act and react upon one another. The body, for example, produces thoughts (sense-impressions) in the soul, the soul produces movements in the body. This would be impossible if both were not of the same substance. The corporeal cannot act on the incorporeal, nor the incorporeal on the corporeal. There is no point of contact. Hence all must be equally corporeal.