We direct our attention to mathematics, natural science, and philosophy, for each of these holds out the hope that it will afford us a part of the explanation we desire. Now, taking philosophy first, we find that it is like a monster with many heads, each of which speaks a different language. They are not, indeed, all at variance on the point we are here considering, the significance of the idea of perception. For, with the exception of the Sceptics and the Idealists, the others, for the most part, speak very much in the same way of an _object_ which constitutes the _basis_ of the idea, and which is indeed different in its whole being and nature from the idea, but yet is in all points as like it as one egg is to another. But this does not help us, for we are quite unable to distinguish such an object from the idea; we find that they are one and the same; for every object always and for ever presupposes a subject, and therefore remains idea, so that we recognised objectivity as belonging to the most universal form of the idea, which is the division into subject and object. Further, the principle of sufficient reason, which is referred to in support of this doctrine, is for us merely the form of the idea, the orderly combination of one idea with another, but not the combination of the whole finite or infinite series of ideas with something which is not idea at all, and which cannot therefore be presented in perception. Of the Sceptics and Idealists we spoke above, in examining the controversy about the reality of the outer world. If we turn to mathematics to look for the fuller knowledge we desire of the idea of perception, which we have, as yet, only understood generally, merely in its form, we find that mathematics only treats of these ideas so far as they fill time and space, that is, so far as they are quantities. It will tell us with the greatest accuracy the how-many and the how-much; but as this is always merely relative, that is to say, merely a comparison of one idea with others, and a comparison only in the one respect of quantity, this also is not the information we are principally in search of.