The first and most obvious way is based on motion. It is certain as a matter of sense-observation that some things in this world are in motion. Now whatever is in motion, is moved by something else. For nothing is in motion except in so far as it is in potentiality to the term of its motion. Something moves, on the other hand, in so far as it is in actuality. This is because to move is precisely to bring something from potentiality to actuality; but a thing cannot be brought from potentiality to actuality except by something which is itself in actuality. Thus, something which is actually hot, like fire, makes something which is potentially hot - say wood - to be actually hot: and in this way it moves and alters it. Now it is not possible for the same thing to be, at the same time and in the same respect, in actuality and in potentiality; for what is actually hot cannot simultaneously be potentially hot, though it may simultaneously be potentially cold. So it is impossible that in the same respect and in the same manner anything should be both mover and moved, or that it should move itself. So whatever is in motion, must be moved by something else. Moreover, this something else, if it too is in motion, must itself be moved by something else, and that in turn by yet another thing. But this cannot go on for ever: because if it did there would be no first mover, and consequently no other mover at all, since second movers do not move except when moved by a first mover, just as a stick does not move anything except when
7 moved by a hand. And so we must reach a first mover which is not moved by anything: and this all men think of as God.1 (la, 2,2.)
The First Way is based on motus. I have translated St. Thomas' term by 'motion': but from the text itself it is clear that the Latin word has a wider meaning than the English one, since the example given of motus is the action of fire on wood. 'Change' is perhaps the English word whose natural sense is nearest. Following Aristotle, (Physics E, 226a 23ft) St. Thomas distinguished three kinds of motus: change of quality, change of quantity and change of place. The first is exemplified when a hot body becomes cold, or a white surface becomes black; it is called technically 'alteration'. The second is increase or decrease in size. The third is called by St. Thomas 'local motion': it is the only one which would naturally be called 'motion' in English.