The essence of matter is acting, it is acting itself, in the abstract, thus acting in general apart from all difference of the kind of action: it is through and through causality. On this account it is itself, as regards its existence, not subject to the law of causality, and thus has neither come into being nor passes away, for otherwise the law of causality would be applied to itself. Since now causality is known to us _a priori_, the conception of matter, as the indestructible basis of all that exists, can so far take its place in the knowledge we possess _a priori_, inasmuch as it is only the realisation of an _a priori_ form of our knowledge. For as soon as we see anything that acts or is causally efficient it presents itself _eo ipso_ as material, and conversely anything material presents itself as necessarily active or causally efficient. They are in fact interchangeable conceptions. Therefore the word “actual” is used as synonymous with “material;” and also the Greek κατ᾽ ενεργειαν, in opposition to κατα δυναμιν, reveals the same source, for ενεργεια signifies action in general; so also with _actu_ in opposition to _potentia_, and the English “actually” for “_wirklich_.” What is called space-occupation, or impenetrability, and regarded as the essential predicate of body (_i.e._ of what is material), is merely that _kind of action_ which belongs to _all_ bodies without exception, the mechanical. It is this universality alone, by virtue of which it belongs to the conception of body, and follows _a priori_ from this conception, and therefore cannot be thought away from it without doing away with the conception itself—it is this, I say, that distinguishes it from any other kind of action, such as that of electricity or chemistry, or light or heat. Kant has very accurately analysed this space-occupation of the mechanical mode of activity into repulsive and attractive force, just as a given mechanical force is analysed into two others by means of the parallelogram of forces. But this is really only the thoughtful analysis of the phenomenon into its two constituent parts. The two forces in conjunction exhibit the body within its own limits, that is, in a definite volume, while the one alone would diffuse it into infinity, and the other alone would contract it to a point. Notwithstanding this reciprocal balancing or neutralisation, the body still acts upon other bodies which contest its space with the first force, repelling them, and with the other force, in gravitation, attracting all bodies in general. So that the two forces are not extinguished in their product, as, for instance, two equal forces acting in different directions, or +E and -E, or oxygen and hydrogen in water. That impenetrability and gravity really exactly coincide is shown by their empirical inseparableness, in that the one never appears without the other, although we can separate them in thought.