“From the surfaces of things,” says Epicurus in the first place, “there passes off a constant stream, which cannot be detected by our senses” (for things would in any other case decrease in size) and which is very fine; “and this because, by reason of the counteracting replenishment, the thing itself in its solidity long preserves the same arrangement and disposition of the atoms; and the motion through the air of these surfaces which detach themselves is of the utmost rapidity, because it is not necessary that what is detached should have any thickness;” it is only a surface. Epicurus says, “Such a conception does not contradict our senses, when we take into consideration how pictures produce their effects in a very similar way, I mean by bringing us into sympathy with external things. Therefore emanations, like pictures, pass out from them into us, so that we see and know the forms and colours of things.”[153] This is a very trivial way of representing sense-perception. Epicurus took for himself the easiest criterion of the truth that is not seen, a criterion still in use, namely that it is not contradicted by what we see or hear. For in truth such matters of thought as atoms, the detachment of surfaces, and so forth, are beyond our powers of sight. Certainly we manage to see and to hear something different; but there is abundance of room for what is seen and what is conceived or imagined to exist alongside of one another. If the two are allowed to fall apart, they do not contradict each other; for it is not until we relate them that the contradiction becomes apparent. “Error,” as Epicurus goes on to say on the second point “comes to pass when, through the movement that takes place within us on the conception therein wrought, such a change is effected that the conception can no longer obtain for itself the testimony of perception. There would be no truth, no likeness of our perceptions, which we receive as in pictures or in dreams or in any other way, if there were nothing on which we, as it were, put out our faculty of observation. There would be no untruth if we did not receive into ourselves another movement, which, to be sure, is conformable to the entering in of the conception, but which has at the same time an interruption.”[154] Error is therefore, according to Epicurus, only a displacement of the pictures in us, which does not proceed from the movement of perception, but rather from this, that we check their influence by a movement originating in ourselves; how this interruption is brought about will be shown more fully later on (pp. 290, 300).