The next point to be observed about Aristotle’s conception is that the _relationship of development is between two terms_. The individual must have two aspects: there must be that out of which the development is passing, and that into which it is passing. Aristotle calls these two aspects of development respectively Matter and Form. Every object of nature consists of Form and Matter, and these two terms have passed into history. To Aristotle everything is Matter becoming Form, or, in other words, Form realizing itself in Matter. The tree has its Matter which is becoming Formed, and its Form into which the Matter is growing. The principle which unites the two is development,――the principle of the individual. Matter, then, is the possibility or potentiality of an individual thing――it is the thing given potentially; Form is its actuality or reality. If you emphasize merely the stages in the development, you are regarding merely the _occurrences_; if, however, you emphasize the stages of development as aspects of a unity, you see its _essence_. The relationship of development between two terms thus becomes under Aristotle’s hands _the relation of purpose_. Aristotle calls this self-realization of the essence in phenomena by the technical word _entelechy_, _i. e._ in opposition to the earlier conceptions of nature Aristotle conceived nature teleologically. Teleology or purpose we found Plato using in his second draft of the Ideas, but more as a postulate than as an efficient means of explanation. Aristotle uses teleology as his positive fundamental principle of nature.