Kant's answer to his question was briefly this. Intelligence is essentially synthetic, always supplementing the given by something beyond, instituting relationships, unifying the many, and thus building up concrete totalities. In pure mathematics this is obvious: the process of numeration shows it creating number out of units, and geometry shows elementary propositions leading on to complicated theorems. In abstract physics it is hardly less obvious: there, e.g., the principle of reason and consequent or the persistence of substance are rational and legitimate steps beyond the mere datum. The more important question follows. How are these 'pure' syntheses applicable to real fact? To that Kant replies: They apply, because in all that we call real or objective fact there is a subjective element or constituent. What appears to be purely given, and independent of our perceptions, is a product of perceptual and conceptual conditions,--is constituted by a synthesis in perception, imagination, conception. Our world is a mental growth--not our individual product, but the work of that common mind in which we live and think, and which lives and thinks in us. Anyhow it is not an isolated self-existing un-intelligent world for ever materially outside us--an other world, eternally separate from us; but bone of our bone, flesh of our flesh, the work realised by our great 'elder brother,'--the Idea of human collectivity--the Reason or Spirit in which we are all one soul. It is therefore no unwarranted step on to a foreign property when we apply the categories of thought and forms of sense to determine objective reality: for objective reality has been for ever made, and is now making, objective and reality by the conscious or unconscious syntheses of perception and imagination.