We may speak of visual Sensation, then, as a _limited_ power of _translating_ light. And what relation does visual _Perception_ bear to this Power? Probably the simplest way of expressing it, is to say that it is neither more nor less than the _translation of a translation_. The mind thus construes to itself what the visual sense is every moment busied with expressing in its own special language--the interpretation of movement, into colour, light and shadow. And from these data--these colours, lights and shadows, the mind draws its own inferences. Now these inferences thus drawn from preceding Sense inferences,--limited in range, as we have seen, and defective in analytic power;--these inferences, such as they are, constitute the boasted certainty of eyesight; and of all things apprehended by its means,--all --quæ sunt oculis subjecta fidelibus et quæ Ipse sibi _tradit_ spectator. It needs but a statement of the mode in which our final mind-interpretations are constructed,--of these translated translations,--obscure in grammar and imperfect in vocabulary--to prove how very difficult is the position of the Realist. In view of this Empire of the Unknowable proclaimed by Science over the surest of our perceiving powers, the firmest foundations of our experimental knowledge, Helmholtz suggests that his reader "may feel determined to stick fast to the 'sound common sense' of mankind, and believe his own senses more than physiology." (p. 270.) And such, no doubt, is the conclusion of the matter to the greater part of mankind. But we will in the first place prefer hearing the last word of the physiologist. From page 270 to page 313 of his work, he argues out the great question of _how_ we perceive under the full impression of its vast importance to psychology, metaphysics, and the first principles upon which all science and all reasonings repose. "We have," he says (p. 281), "already learned enough to see that the questions which have here to be decided are of fundamental importance, not only for the physiology of sight, but for a correct understanding of the true nature and limits of human knowledge generally."