One must indeed be forsaken by all the gods, to imagine that the outer, perceptible world, filling Space in its three dimensions and moving on in the inexorable flow of Time, governed at every step by the laws of Causality, which is without exception, and in all this merely obeying laws we can indicate before all experience of them--that such a world as this, we say, can have a real, objective existence outside us, without any agency of our own, and that it can then have found its way into our heads through bare sensation and thus have a second existence within us like the one outside. For what a miserably poor thing is mere sensation, after all! Even in the noblest of our organs it is nothing but a local, specific feeling, susceptible of some slight variation, still in itself always subjective and, as such therefore, incapable of containing anything objective, anything like perception. For sensation is and remains a process within the organism and is limited, as such, to the region within the skin; it cannot therefore contain anything which lies beyond that region, or, in other words, anything that is outside us. A sensation may be pleasant or unpleasant--which betokens a relation to the Will--but nothing objective can ever lie in any sensation. In the organs of the senses, sensation is heightened by the confluence of the nerve-extremities, and can easily be excited from without on account of their extensive distribution and the delicacy of the envelope which encloses them; it is besides specially susceptible to particular influences, such as light, sound, smell; notwithstanding which it is and remains mere sensation, like all others within our body, consequently something essentially subjective, of whose changes we only become immediately conscious in the form of the _inner_ sense, Time: that is, successively. It is only when the _Understanding_ begins to act--a function, not of single, delicate nerve-extremities, but of that mysterious, complicated structure weighing from five to ten pounds, called the brain--only when it begins to apply its sole form, _the causal law_, that a powerful transformation takes place, by which subjective sensation becomes objective perception. For, in virtue of its own peculiar form, therefore _à priori_, _i.e._ _before_ all experience (since there could have been none till then), the Understanding conceives the given corporeal sensation as an _effect_ (a word which the Understanding alone comprehends), which _effect_, as such, necessarily implies a _cause_. Simultaneously it summons to its assistance _Space_, the form of the _outer_ sense, lying likewise ready in the intellect (_i.e._ the brain), in order to remove that cause _beyond_ the organism; for it is by this that the external world first arises, Space alone rendering it possible, so that pure intuition _à priori_ has to supply the foundation for empirical perception. In this process, as I shall soon show more clearly, the Understanding avails