It was with the help of this theory that Epicurus explained and defended the current belief in the existence of gods. The divine inhabitants of the _intermundia_, or empty spaces separating world from world, are, like all other beings, composed of atoms, and are continually throwing off fine images, some of which make their way unaltered to our earth and reveal themselves to the senses, particularly during sleep, when we are most alive to the subtlest impressions on our perceptive organs. With the usual irrationality of a theologian, Epicurus remained blind to the fact that gods who were constantly throwing off even the very thinnest films could not possibly survive through all eternity. Neither did he explain how images larger than the pupil of the eye could pass through its aperture while preserving their original proportions unaltered. We have seen how Epicurus erected the senses into ultimate arbiters of truth. By so doing, however, he only pushed the old difficulty a step further back. Granting that our perceptions faithfully correspond to certain external images, how can we be sure that these images are themselves copies of a solid and permanent reality? And how are we to determine the validity of general notions representing not some single object but entire classes of objects? The second question may be most conveniently answered first. Epicurus holds that perception is only a finer sort of sensation. General notions are material images of a very delicate texture formed, apparently, on the principle of composition-photographs by the coalescence of many individual images thrown off from objects possessing a greater or less degree of resemblance to one another.[186] Thought is produced by the contact of such images with the soul, itself, it will be remembered, a material substance. The rules for distinguishing between truth and falsehood are given in the famous Epicurean Canon. On receiving an image into the mind, we associate it with similar images formerly impressed on us by some real object. If the association or anticipation (πρόληψις) is confirmed or not contradicted by subsequent experience, it is true; false, if contradicted or not confirmed.[187] The stress laid on absence of contradictory evidence illustrates the great part played by such notions as possibility, negation, and freedom in the Epicurean system. In ethics this class of conceptions is represented by painlessness, conceived first as the condition, and finally as the essence of happiness; in physics by the infinite void, the _inane profundum_ of which Lucretius speaks with almost religious unction; and in logic by the absence of contradiction considered as a proof of reality. Here, perhaps, we may detect the Parmenidean absolute under a new form; only, by a curious reversal, what Parmenides himself strove altogether to expel from thought has become its supreme object and content.[188]