Plato was, perhaps, the first to attempt to prove the existence of God from the essential principles of knowledge. He could not consistently reason from the impressions of sense or the phenomena of the visible world. He denied that sense is knowledge, and that visible things can be more than images and indications of truth. He maintained, however, that besides the visible world there is an intelligible world, with objects which reason sees and not sense. These objects are either conceptions or ideas, either hypothetical principles or absolute principles, either scientific assumptions and definitions or necessary and eternal truths which have their reality and evidence in themselves. The mathematical sciences deal with conceptions; but their chief value, according to Plato, is that they help the mind to rise to that absolute science--dialectics--which is conversant with ideas. The apprehension of ideas is the apprehension of the common element in the manifold, the universal in the individual, the permanent in the mutable. Reason contemplates ideas, and participates in ideas, and ideas are at once the essences of things and the regulative principles of cognition. By communion with them the reason reaches objective reality and possesses subjective certainty. They are not isolated and unconnected, but so related that each higher idea comprehends within it several lower ones, and that all combined constitute a graduated series or articulated organism, unified and completed by an idea which has none higher than itself, which is ultimate, which conditions all the others while it is conditioned by none. The supreme idea, which contains in itself all other ideas, is absolute truth, absolute beauty, absolute good, absolute intelligence, and absolute being. It is the source of all true existence, knowledge, and excellence. It is God. In this part of its course the dialectic of Plato is simply a search for God. It is _a priori_ inasmuch as it rests on necessary ideas, but _a posteriori_ inasmuch as it proceeds from these ideas upwards to God in a manner which is essentially analytic and inductive. Only when God--the principle of principles--is reached, can it become synthetic and deductive.