Note 168 — POSITIVISM.
Reading edition
"Does any one fancy that he sees a solid cube? It is easy to show that the solidity of the figure, the relative position of its faces and edges to each other, are inferences of the spectator—no more conveyed to his conviction by the eye alone than they would be if he were looking at a painted representation of a cube. The scene of nature is a picture without depth of substance, no less than the scene of art; and in the one case, as in the other, it is the mind which, by an act of its own, discovers that colour and shape denote distance and solidity. Most men are unconscious of this perpetual habit of reading the language of the external world, and translating as they read. The draughtsman, indeed, is compelled, for his purposes, to return back in thought from the solid bodies which he has inferred, to the shapes of surface which he really sees. He knows that there is a mask of theory over the whole face of nature, if it be theory to infer more than we see. But other men, unaware of this masquerade, hold it to be a fact that they see cubes and spheres, spacious apartments, and winding avenues. And these things are facts to them, because they are unconscious of the mental operation by which they have penetrated nature's disguise....
"Our sensations require ideas to bind them together; namely, ideas of space, time, number, and the like. If not so bound together, sensations do not give us any apprehension of things or objects. All things, all objects, must exist in space and in time—must be one or many. Now space, time, number, are not sensations or things. They are something different from, and opposed to, sensations and things. We have termed them ideas. It may be said they are relations of things, or of sensations. But granting this form of expression, still a relation is not a thing or a sensation; and therefore we must still have another and opposite element, along with our sensations....
"We are often told that such a thing is a fact—a fact, and not a theory,—with all the emphasis which, in speaking or writing, tone or italics or capitals can give. We see from what has been said, that when this is urged, before we can estimate the truth, or the value of the assertion, we must ask to whom is it a fact? what habits of thought, what previous information, what ideas does it imply, to conceive the fact as a fact? Does not the apprehension of the fact imply assumptions which may with equal justice be called theory, and which are perhaps false theory? in which case the fact is no fact. Did not the ancients assert it as a fact, that the earth stood still, and the stars moved? and can any fact have stronger apparent evidence to justify persons in asserting it emphatically than this had?"—Whewell's Philosophy of the Inductive Sciences, 2nd ed., vol. i., p. 42, seq.
That the solidity of figures is in truth given by mental judgment, has been often proved experimentally; see for examples, Huxley's Elementary Physiology, Lesson x., 13–16. The experiment with a coin, lens and pin, p. 259, is easy as well as conclusive, but Wheatstone's Pseudoscope more surprising to most observers. Compare on this curious subject Brewster's Natural Magic, Letter v.