What is the end of the path of a snow-flake? St Thomas would probably have said that its end was to find the proper place of water, which is below air and above earth. But in this chapter he returns a more scientific answer: "Every agent tends to some definite effect, and that is called its end." Speaking then of physical agencies, and remaining in the mere physical order, all that the statement at the head of this chapter comes to is this, that physical agents work definitely and uniformly: or as Aristotle puts it (De part. animal. I, I, 34, 37, 38): "Nature does all things for something: . . . . everywhere we say, this for that, wherever there appears an end in which the motion terminates, provided nothing comes in the way: wherefore it is evident that there is such a thing as what we call nature: for it is not anything and everything that comes out of every germ, but this out of that."
Of God and His Creatures: 3.2