Without application of the law of causality, however, perception of an _objective_ world could never be arrived at; 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, 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 sense 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._, 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.