The World as Will and Idea, Vol. II — Arthur Schopenhauer, trans. Haldane & Kemp

With all transcendental ideality the objective world retains empirical reality ; the object is indeed not the thing in itself, but as an empirical object it is real. It is true that space is only in my head ; but empirically my head is in space. The law of causality can certainly never enable us to get quit of idealism by building a bridge between things in themselves and our knowledge of them, and thus certifying the absolute reality of the world, which exhibits itself in consequence of its application ; but this by no means does away with the causal relation of objects to each other, thus it does not abolish the causal relation which unquestionably exists between the body of each knowing person and all other material objects. But the law of causality binds together only phenomena, and does not lead beyond them. With that law we are and remain in the world of objects, i.e., the world of phenomena, or more properly the world of ideas. Yet the whole of such a world of experience is primarily conditioned by the knowledge of a subject in general as its necessary presupposition, and then by the special forms of our perception and apprehension, thus necessarily belongs to the merely phenomenal, and has no claim to pass for the world of things in themselves. Indeed the subject itself (so far as it is merely the knowing subject) belongs to the merely phenomenal, of which it constitutes the complementary half.

Without application of the law of causality, however, perception of an objective world could never be arrived at ;

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for this perception is, as I have often explained, essentially matter of the intellect, and not merely of the senses. The senses afford us mere sensation, which is far from being perception. The part played by sensations of the senses in perception was distinguished by Locke under the name secondary qualities, which he rightly refused to ascribe to things in themselves.

But Kant, carrying Locke's method further, distinguished also, and refused to ascribe to things in themselves what belongs to the working up of this material (the sensations) by the brain. The result was, that in this was included all that Locke had left to things in themselves as primary qualities — extension, form, solidity, &c. — so that with Kant the thing in itself was reduced to a completely unknown quantity = x.

With Locke accordingly the thing in itself is certainly without colour, sound, smell, taste, neither warm nor cold, neither soft nor hard, neither smooth nor rough ; yet it has still extension and form, it is impenetrable, at rest or in motion, and has mass and number.

With Kant, on the other hand, it has laid aside all these latter qualities also, because they are only possible by means of time, space, and causality, and these spring from an intellect (brain), just as colours, tones, smells, &c, originate in the nerves of the organs of sense. The thing in itself has with Kant become spaceless, unextended, and incorporeal.

Thus what the mere senses bring to the perception, in which the objective world exists, stands to what is supplied by the functions of the brain (space, time, causality) as the mass of the nerves of sense stand to the mass of the brain, after subtracting that part of the latter which is further applied to thinking proper, i.e., to abstract ideas, and is therefore not possessed by the brutes.

For as the nerves of the organs of sense impart to the phenomenal objects colour, sound, taste, smell, temperature, &c, so the brain imparts to them extension, form, impenetrability, the power of movement, &c, in short all that can only be presented in perception by means of time, space,

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and causality. How small is the share of the senses in perception, compared with that of the intellect, is also shown by a comparison of the nerve apparatus for receiving impressions with that for working them up. The mass of the nerves of sensation of the whole of the organs of senso is very small compared with that of the brain, even in the case of the brutes, whose brain, since they do not, properly speaking, i.e.t in the abstract, think, is merely used for effecting perception, and yet when this is complete, thus in the case of mammals, has a very considerable mass, even after the cerebellum, whose function is the systematic guidance of movements, has been taken away.