experience of change must be thinkable, rational. To the degree the universe is indeed thinkable, there is an implication of purpose to change, whether natural or artificial. And if there is purpose in the universe, then it becomes possible to discover principles by which both the actuality and potentiality of the universe can be explained. In explaining the shortcomings of the natural philosophy of such predecessors as Empedocles and Pythagoras, Aristotle declares:
It is plain then that they all in one way or another identify the contraries with the principles. And with good reason. For first principles must not be derived from one another nor from anything else, while everything has to be derived from them. But these conditions are fulfilled by the primary contraries, which are not derived from anything else because they are primary, nor from each other because they are contraries.
But we must see how this can be arrived at as a reasoned result. Our first presupposition must be that in nature nothing acts on, or is acted on by, any other thing at r
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