Only where all _a priori_ assertions cease, therefore in the _entirely empirical_ part of our knowledge of bodies, in their form, quality, and definite manner of acting, does that _will_ reveal itself which we have already recognised and established as the true inner nature of things. But these forms and qualities always appear only as the properties and manifestations of that very _matter_ the existence and nature of which depends upon the subjective forms of our intellect, _i.e._, they only become visible in it, and therefore by means of it. For that which always exhibits itself to us is only _matter_ acting in some specially determined manner. Out of the inner properties of such matter, properties which cannot be further explained, proceeds every definite kind of effect of given bodies; and yet the matter itself is never perceived, but only these effects, and the definite properties which lie at their foundation, after separating which, matter, as that which then remains over, is necessarily added in thought by us; for, according to the exposition given above, it is objectified _causality itself_. Accordingly matter is that whereby the _will_, which constitutes the inner nature of things, becomes capable of being apprehended, perceptible, _visible_. In this sense, then, matter is simply the _visibility_ of the will, or the bond between the world as will and the world as idea. It belongs to the latter inasmuch as it is the product of the functions of the intellect, to the former inasmuch as that which manifests itself in all material existences, _i.e._, phenomena is the will. Therefore every object is, as thing in itself, will, and as phenomenon, matter. If we could strip any given matter of all the properties that come to it _a priori_, _i.e._, of all the forms of our perception and apprehension, we would have left the thing in itself, that which, by means of those forms, appears as the purely empirical in matter, but which would then itself no longer appear as something extended and active; _i.e._, we would no longer have matter before us, but the will. This very thing in itself, or the will, in that it becomes a phenomenon, _i.e._, enters the forms of our intellect, appears as matter, _i.e._, as the invisible but necessarily assumed supporter of the properties which are only visible through it. In this sense, then, matter is the visibility of the _will_. Consequently Plotinus and Giordano Bruno were right, not only in their sense but also in ours, when they made the paradoxical assertion already referred to in chapter 4: Matter itself is not extended, consequently it is incorporeal. For space, which is our form of perception, imparts extension to matter, and corporeal existence consists in acting, which depends upon causality, and consequently upon the form of our understanding. On the other hand, every definite property, thus everything empirical in matter, even gravity, depends upon that which only