Matter has already been spoken of in the fourth chapter of the supplements to the first book, when we were considering the part of our knowledge of which we are conscious _a priori_. But it could only be considered there from a one‐sided point of view, because we were then concerned merely with its relation to the forms of our intellect, and not to the thing in itself, and therefore we investigated it only from the subjective side, _i.e._, so far as it is an idea, and not from the objective side, _i.e._, with regard to what it may be in itself. In the first respect, our conclusion was that it is objective _activity_ in general, yet conceived without fuller determination; therefore it takes the place of causality in the table of our _a priori_ knowledge which is given there. For what is material is that which _acts_ (the actual) in general, and regarded apart from the specific nature of its action. Hence also matter, merely as such, is not an object of _perception_, but only of _thought_, and thus is really an abstraction. It only comes into perception in connection with form and quality, as a body, _i.e._, as a fully determined kind of activity. It is only by abstracting from this fuller determination that we think of matter as such, _i.e._, separated from form and quality; consequently under matter we think of _acting_ absolutely and in general, thus of _activity_ in the abstract. The more fully determined acting we then conceive as the _accident_ of matter; but only by means of this does matter become perceptible, _i.e._, present itself as a body and an object of experience. Pure matter, on the other hand, which, as I have shown in the Criticism of the Kantian Philosophy, alone constitutes the true and admissible content of the conception of _substance_, is causality itself, thought objectively, consequently as in space, and therefore filling it. Accordingly the whole being of matter consists in _acting_. Only thus does it occupy space and last in time. It is through and through pure causality. Therefore wherever there is action there is matter, and the material is the active in general. But causality itself is the form of our understanding; for it is known to us _a priori_, as well as time and space. Thus matter also, _so far_ and up to this point, belongs to the formal part of our knowledge, and is consequently that form of the understanding, _causality_ itself, bound up with space and time, hence objectified, _i.e._, conceived as that which fills space. (The fuller explanation of this doctrine will be found in the second edition of the essay on the principle of sufficient reason, p. 77; third edition, p. 82.) So far, however, matter is properly not the _object_ but the _condition_ of experience; like the pure understanding itself, whose function it so far is. Therefore of pure matter there is also only a _conception_, no _perception_. It enters into all external experience as a necessary