Next to its bearing on the question of immortality, the Epicurean psychology is most interesting as a contribution to the theory of cognition. Epicurus holds that all our knowledge is derived from experience, and all our experience, directly or indirectly, from the presentations of sense. So far he says no more than would be admitted by the Stoics, by Aristotle, and indeed by every Greek philosopher except Plato. There is, therefore, no necessary connexion between his views in this respect and his theory of ethics, since others had combined the same views with a very different standard of action. It is in discussing the vexed question of what constitutes the ultimate criterion of truth that he shows to most disadvantage in comparison with the more intellectual schools. He seems to have considered that sensation supplies not only the matter but the form of knowledge; or rather, he seems to have missed the distinction between matter and form altogether. What the senses tell us, he says, is always true, although we may draw erroneous inferences from their statements.[184] But this only amounts to the identical proposition that we feel what we feel; for it cannot be pretended that the order of our sensations invariably corresponds to the actual order of things in themselves. Even confining ourselves to individual sensations, or single groups of sensations, there are some that do not always correspond to the same objective reality, and others that do not correspond to any reality at all; while, conversely, the same object produces a multitude of different sensations according to the subjective conditions under which it affects us. To escape from this difficulty, Epicurus has recourse to a singularly crude theory of perception, borrowed from Empedocles and the older atomists. What we are conscious of is, in each instance, not the object itself, but an image composed of fine atoms thrown off from the surfaces of bodies and brought into contact with the organs of sense. Our perception corresponds accurately to an external image, but the image itself is often very unlike the object whence it originally proceeded. Sometimes it suffers a considerable change in travelling through the atmosphere. For instance, when a square tower, seen at a great distance, produces the impression of roundness, this is because the sharp angles of its image have been rubbed off on the way to our eyes. Sometimes the image continues to wander about after its original has ceased to exist, and that is why the dead seem to revisit us in our dreams. And sometimes the images of different objects coalesce as they are floating about, thus producing the appearance of impossible monsters, such as centaurs and chimaeras.[185]