§ 43. Matter as such cannot be the expression of an Idea. For, as we found in the first book, it is throughout nothing but causality: its being consists in its casual action. But causality is a form of the principle of sufficient reason; knowledge of the Idea, on the other hand, absolutely excludes the content of that principle. We also found, in the second book, that matter is the common substratum of all particular phenomena of the Ideas, and consequently is the connecting link between the Idea and the phenomenon, or the particular thing. Accordingly for both of these reasons it is impossible that matter can for itself express any Idea. This is confirmed _a posteriori_ by the fact that it is impossible to have a perceptible idea of matter as such, but only an abstract conception; in the former, _i.e._, in perceptible ideas are exhibited only the forms and qualities of which matter is the supporter, and in all of which Ideas reveal themselves. This corresponds also with the fact, that causality (the whole essence of matter) cannot for itself be presented perceptibly, but is merely a definite casual connection. On the other hand, _every phenomenon_ of an Idea, because as such it has entered the form of the principle of sufficient reason, or the _principium individuationis_, must exhibit itself in matter, as one of its qualities. So far then matter is, as we have said, the connecting link between the Idea and the _principium individuationis_, which is the form of knowledge of the individual, or the principle of sufficient reason. Plato is therefore perfectly right in his enumeration, for after the Idea and the phenomenon, which include all other things in the world, he gives matter only, as a third thing which is different from both (Timaus, p. 345). The individual, as a phenomenon of the Idea, is always matter. Every quality of matter is also the phenomenon of an Idea, and as such it may always be an object of æsthetic contemplation, _i.e._, the Idea expressed in it may always be recognised. This holds good of even the most universal qualities of matter, without which it never appears, and which are the weakest objectivity of will. Such are gravity, cohesion, rigidity, fluidity, sensitiveness to light, and so forth.