On the other hand, the controversy concerning the reality of the external world considered in § 5 of the first volume rests upon the assumption, which has just been criticised, of an objective and a subjective world both in _space_, and upon the impossibility which arises in connection with this presupposition of a transition from one to the other, a bridge between the two. Upon this controversy I have still to add the following remarks. The subjective and the objective do not constitute a continuous whole. That of which we are immediately conscious is bounded by the skin, or rather by the extreme ends of the nerves which proceed from the cerebral system. Beyond this lies a world of which we have no knowledge except through pictures in our head. Now the question is, whether and how far there is a world independent of us which corresponds to these pictures. The relation between the two could only be brought about by means of the law of causality; for this law alone leads from what is given to something quite different from it. But this law itself has first of all to prove its validity. Now it must either be of _objective_ or of _subjective_ origin; but in either case it lies upon one or the other side, and therefore cannot supply the bridge between them. If, as Locke and Hume assume, it is _a posteriori_, thus drawn from experience, it is of _objective_ origin, and belongs then itself to the external world which is in question. Therefore it cannot attest the reality of this world, for then, according to Locke’s method, causality would be proved from experience, and the reality of experience from causality. If, on the contrary, it is given _a priori_, as Kant has more correctly taught us, then it is of _subjective_ origin, and in that case it is clear that with it we remain always in the _subjective_ sphere. For all that is actually given _empirically_ in perception is the occurrence of a sensation in the organ of sense; and the assumption that this, even in general, must have a cause rests upon a law which is rooted in the form of our knowledge, _i.e._, in the functions of our brain. The origin of this law is therefore just as subjective as that of the sensation itself. The cause of the given sensation, which is assumed in consequence of this law, presents itself at once in perception as an _object_, which has space and time for the form of its manifestation. But _these forms_ themselves again are entirely of subjective origin; for they are the mode or method of our faculty of perception. That transition from the sensation to its cause which, as I have repeatedly pointed out, lies at the foundation of all sense-perception is certainly sufficient to give us the empirical presence in space and time of an empirical object, and is therefore quite enough for the practical purposes of life; but it is by no means sufficient to afford us any conclusion as to the existence and real nature, or rather as