Appendix 30.

(846) [Extension misunderstood]

Philosophy has come down to us not through one but many channels, and has nearly come to grief in the limitless ocean of modern scepticism. I have traced the history of this system (which is really a negation of system) through Locke, Berkeley, Hume, Reid and Kant, as well as through Condillac and the French sceptics. The same philosophical destruction came to us through another channel: Descartes, Bayle and Kant. Descartes, who had made famous and universally acceptable Galileo's opinion that the secondary properties of bodies were only in the subject, posited the essence of bodies in extension. His error consisted in his failure to observe that all our sensations (colour, taste, sound, odour, etc.), although subjective, necessarily contain an extrasubjective part.

When Bayle came on the scene, this extrasubjective part had been forgotten, and all the above-mentioned sensations were taken as subjective. Bayle applied the arguments which Descartes had used for secondary qualities and showed that primary qualities, one of which was extension, were subjective. His argument was simple in the extreme and ad hominem: we perceive extension only through a sensation; but sensations are subjective, therefore extension is subjective.

Kant, beginning from this point, needed only to invent the name 'form of external sense' for the subject's aptitude for perceiving space. He had now entered the domain of critical philosophy. He took a few steps in this territory, on to which he had been thrown, as it were, by the shipwreck of his time, and found himself on the sad terrain of transcendental philosophy. After Descartes' small error in missing the extrasubjective element intermingled with all our subjective sensations, extension could no longer be defended.