At the end of Book I of Aristotle’s Physics, he claims that he has sufficiently argued that there are principles, that he has shown what those principles are, and that he has determined how many of them there are. Why should we care about principles? Because principles are the doors to scientific knowledge. Aristotle draws on his predecessors, who all agree that contraries are principles. Examples of contraries include excess and defect, above and below, before and behind, angular and angle-less, straight and round, and so forth. If contraries come in pairs and contraries are principles, then in the abstract, there are 2 principles. As we observe things in our world, we notice that their properties tend to change between the contraries. Things change from hot to cold, wet to dry, e.t.c and vice versa. The things in which the contraries display their properties and which persist through the change is called the substratum, which is the third principle.
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