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The point of departure, then, lies in the philosophical line. Little as he could foresee the future influence of his theory, Locke raised, in a manner, the entire question of the relation between consciousness and the physiological organism by his famous distinction between the primary and secondary qualities of body.[1] Qualities like color, odor, hardness and sound, he called secondary, because they can not become effective components of consciousness unless the appropriate organs cooperate. Neither color nor sound resides in nature, but motions of such and such amplitude. For us, therefore, color and sound happen to be interpretations by eye and ear of something incommensurable with the perceptions in consciousness. On the contrary, qualities such as resistance and extension belong to objects in their own right, and persist independent of any cooperation by our sense organs. Locke did not grasp the philosophical problems, involved here, much less the extreme complexity of the physiological processes he assumed. However, he does advert to one of the difficulties embedded in his view—the "mystery," as it remains even yet, of space perception:

I shall here insert a problem of that very ingenious and studious promoter of real knowledge, the learned and worthy Mr. Molineux, which he was pleased to send me in a letter some months since; and it is this:—"Suppose a man born blind, and now adult, and taught by his touch to distinguish between a cube and a sphere of the same metal, and nighly of the same bigness, so as to tell, when he felt one and the other, which is the cube, which the sphere. Suppose then the cube and the sphere placed on a table, and the blind man be made to see:quaere, whether by his sight, before he touched them, he could now distinguish and tell which is the globe, which the cube?" To which the acute and judicious proposer answers, "Not. For, though he has obtained the experience of how a globe, how a cube affects his touch, yet he has not yet obtained the experience that what affects his touch so or so, must affect his sight so or so; or that a protuberant angle in the cube, that pressed his hand unequally, shall appear to his eye as it does in the cube."—I agree with this thinking gentleman, whom I am proud to call my friend, in his answer to this problem; and am of opinion that the blind man, at first sight, would not be able with certainty to say which was the globe, which the cube, whilst he only saw them; though he could unerringly name them by his touch, and certainly distinguish them by the difference of their figures felt. This I have set down, and leave with my reader, as an occasion for him to consider how much he may be beholden to experience, improvement and acquired notions, where he thinks he had not the least use of, or help from them.[2]

As the last sentence indicates, this reference remains incidental rather than determining for Locke.

It was left for his successor and critic Berkeley to give special form to the problem for its own sake, in his "Essay towards a New Theory of Vision" (1709). With remarkable prescience, he writes:

Rightly to conceive the business in hand, we must carefully distinguish
  1. "Essay concerning Human Understanding," Bk. II.; Chap. VIII.
  2. "Essay concerning Human Understanding," Bk. II., Chap. IX., Sect. 8.