[#] Self-evident, that is, in a purely logical sense. When you apprehend the principles in question, you _see_ at once that they are true, and do not require to have them _proved_. It is not meant that any and every man _does_, in point of fact, always apprehend the principles, or that they can be apprehended without preliminary mental discipline. We proceed to the question how many subdivisions there are within "theoretical" Philosophy itself. Plato had held that there are none. All the sciences are deductions from a single set of ultimate principles which it is the business of that supreme science to which Plato had given the name of Dialectic to establish. This is not Aristotle’s view. According to him, "theoretical" Philosophy falls into a number of distinct though not co-ordinate branches, each with its own special subjects of investigation and its own special axiomatic principles. Of these branches there are three, First Philosophy, Mathematics, and Physics. First Philosophy--afterwards to be known to the Middle Ages as Metaphysics[#]--treats, to use Aristotle’s own expression, of "Being _quà_ Being." This means that it is concerned with the universal characteristics which belong to the system of knowable reality as such, and the principles of its organisation in their full universality. First Philosophy alone investigates the character of those causative factors in the system which are without body or shape and exempt from all mutability. Since in Aristotle’s system God is the supreme Cause of this kind, First Philosophy culminates in the knowledge of God, and is hence frequently called Theology. It thus includes an element which would to-day be assigned to the theory of knowledge, as well as one which we should ascribe to metaphysics, since it deals at once with the ultimate postulates of knowledge and the ultimate causes of the order of real existence. [#] The origin of this name seems to be that Aristotle’s lectures on First Philosophy came to be studied as a continuation of his course on Physics. Hence the lectures got the name _Metaphysica_ because they came _after_ (_meta_) those on Physics. Finally the name was transferred (as in the case of _Ethics_) from the lectures to the subject of which they treat.