=The Fundamental Principle in Aristotle’s Philosophy.= The first question then is, How did Aristotle reëstablish the perceptual fact? What means did he employ to give the perceptual fact a reality? The answer to this question will be the statement of Aristotle’s fundamental principle. It will show his advance over Plato by showing his new estimate of the perceptual world. Plato accepted the Protagorean doctrine of perception, but also gave it a new value by placing perceptions beside conceptions in the world of reality; Aristotle developed Plato’s teaching about perceptions by linking them inseparably with conceptions. Aristotle felt that Plato’s difficulties arose from the lack of close relationship between conceptual Being and perceptual fact. What is that linkage? What binds abiding reality and changing phenomena so closely? _The linkage is development._ Development is the relation between conception and perception. It is the fundamental principle in the philosophy of Aristotle throughout and places a new estimate upon the value of perception. Perceptual facts apart from conceptions have no reality; conceptions apart from perceptions are mere abstractions. In the world of reality conceptual Being resides in the perceptual facts, and the perceptual facts express conceptions. They always exist together in a linkage or relationship that is teleological, purposeful――the linkage of development. An abstract statement of this relationship is, “Aristotle felt the conceptual necessity of the empirically actual.” Perhaps the clearest statement of this fundamental principle can be made in the terms of evolution. It is this: _true reality is the essence which unfolds in phenomena_. Notice that this sentence has two parts equally freighted: _reality is an unfolding essence_; _reality is in phenomena_. The true universal must be thought as realizing itself through its development in particulars; the true concept as realizing itself through its development in percepts; the true abiding Being as realizing itself in its development through change. On the one hand, reality is the essence of things; on the other, reality has existence only in things. True reality is the individual. The individual consists of two aspects: (1) conceptual being, and (2) perceptual change. These two aspects always stand in a relationship. That relationship is developing purpose.