Without entering into a long discussion it is sufficient for our present purpose of reconciling the laguage of metaphysics with that of common sense to remark that we have good reason to attribute to ourselves, rather than to anything else, the phenomena which we express the most perfectly, and that we attribute to other substances those phenomena which each one expresses the best. Thus a substance, which is of an infinite extension in so far as it expresses all, becomes limited in proportion to its more or les perfect manner of expression. It is thus then that we may conceive of substances as interfering with and limiting one ‘another, and hence we are able to say that in this sense they act upon one another, and that they are obliged, so to speak, to accomodate themselves to one another. For it can happen that a single change which augments the expression of the one may diminish that of the other. Now the virtue of a particular substance is to express well the glory of God, and the better it expresses it the less it is limited. Everything, when it expresses its virtue of power, that is to say when it acts, changes to better and expands just in so far as it acts. When therefore a change occurs by which several substances are affected (in fact every change affects them all) I think we may say that those substances, which by this change pass immediately to a greater degree of perfection or to a more perfect expression, exert power and act, while those which pass to a lesser degree disclose their weakness and are passive. I also hold that every activity of a substance which has perception implies some pleasure, and every passivity some pain, and vice versa; nevertheless it may very well happen that a present advantage will be eventually destroyed by a greater evil, whence it comes that one may sin in acting or exerting his power and in finding pleasure.
Chapter: 15