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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...
