Knowledge, in fine, may be regarded as an ascending series of four terms. The first is constituted by sensations associated together in such a way that a relation of antecedence and consequence exists between them. This is "experience." The second stage comes into existence when we connect these experiences, not by mere relations of "consecution," but by their conditions, by the principle of causality, and especially by that of sufficient reason, which connects them with the supreme intelligence, God. This stage is science. The third is knowledge of the axioms and necessary truths in and of themselves, not merely as involved in science. The fourth is self-consciousness, the knowledge of intelligence, in its intimate and universal nature, by which we know God, the mind, and all real substance. In the order of time the stage of experience is first, and that of self-consciousness last. But in the lowest stage there are involved the others. The progress of knowledge consists in the development or unfolding of this implicit content, till intelligence, spirit, activity, is clearly revealed as the source and condition of all. CHAPTER XI. THE THEOLOGY OF LEIBNIZ. One of the chapters concerning knowledge is entitled, "The Knowledge that we have of God." This introduces us to the theology of Leibniz and indirectly to the completion of those ethical doctrines already outlined in the chapter on will. Leibniz employs three arguments to prove the existence of God: that of God as the sufficient reason of the world (substantially the cosmological proof); of God as the source of the pre-established harmony (an extension of the teleological proof); and the ontological. The latter he accepts as it came from the hands of Descartes, but insists that it requires an added argument before it ranks as anything more than presumptive proof. The Anselmic-Cartesian argument, as stated by Leibniz, is as follows: "God is defined as the greatest, or most perfect, of beings, or as a being of supreme grandeur and perfection. But in the notion of a perfect being, existence must be included, since it is something more to exist than not to exist. Or existence is a perfection, and hence must belong to the most perfect being; otherwise some perfection would be lacking, which is contrary to the definition." Or as Descartes sometimes puts it, in the notion of anything like a tree, a mountain, a triangle, contingency is contained. We may conceive such an object to exist or not, as we like. There is no necessity involved in our thought. But we cannot think of a perfect being except as existing. It does not rest with the decision of our thinking whether or not to include existence in this notion. We must necessarily think existence as soon as we think such a being.