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That Principles of Nature Must Be Contraries

A friend wrote with a good question:

I am reading Chapter 6 of [Aristotle’s] Physics. His main point appears to be to confirm that the first principles are contaries. He says all of the philosophers thought the first principles were contraries in some manner. He then explains at the end of this chapter the idea that universals are objects of reason, like the great and the small, while particulars, like the rare and the dense are known by sense. Then he says “That it is necessary therefore that principles should be contraries is evident.

I feel like I am missing something here. Why is it “necessary”? Why must the first principles be contaries and how does this follow from his reasonsing?

Aristotle explains the reason why contrariety is necessary back up near the beginning of the chapter. He writes:

“That all philosophers, therefore, in a certain respect make principles to be contraries, is manifest. And it is reasonable that they should, for it is necessary that principles should neither be produced from each other, nor from other things, and that from these all things should be generated. But these requisites are inherent in the first contraries. For, because they are first, they are not from other things, and because they are contraries, they are not from each other.”

After explaining this in detail, he concludes the chapter with his general point:

“That it is necessary therefore that principles should be contraries is evident.”

Aquinas explains:

“Three things seem to belong to the very nature of principles. First, they are not from other things. Secondly, they are not from each other. Thirdly, all other things are from them. But these three notes are found in the primary contraries. Therefore the primary contraries are principles.

Now in order to understand what he means when he speaks of primary contraries, we must realize that some contraries are caused by other contraries, e.g., the sweet and the bitter are caused by the wet and the dry and the hot and the cold. Since, however, it is impossible to proceed to infinity, but one must come to certain contraries, which are not caused by other contraries, he calls these last contraries the primary contraries.

Now the three conditions proper to principles mentioned above are found in these primary contraries. For things which are first are manifestly not from others. Moreover things which are contraries are manifestly not from each other. For even though the cold comes to be from the hot, insofar as that which was previously hot is later cold, nevertheless coldness itself never comes to be from heat, as will be pointed out later. The third point—precisely how all things come to be from the contraries -we must investigate more carefully.


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