circle; and there may be motion in different places, having sometimes (2) one centre of motion and sometimes (3) more. (4) When bodies in motion come against other bodies which are at rest, they are divided by them, and (5) when they are caught between other bodies coming from opposite directions they unite with them; and (6) they grow by union and (7) waste by dissolution while their constitution remains the same, but are (8) destroyed when their constitution fails. There is a growth from one dimension to two, and from a second to a third, which then becomes perceptible to sense; this process is called generation, and the opposite, destruction. We have now enumerated all possible motions with the exception of two. 'What are they?' Just the two with which our enquiry is concerned; for our enquiry relates to the soul. There is one kind of motion which is only able to move other things; there is another which can move itself as well, working in composition and decomposition, by increase and diminution, by generation and destruction. 'Granted.' (9) That which moves and is moved by another is the ninth kind of motion; (10) that which is self-moved and moves others is the tenth. And this tenth kind of motion is the mightiest, and is really the first, and is followed by that which was improperly called the ninth. 'How do you mean?' Must not that which is moved by others finally depend upon that which is moved by itself? Nothing can be affected by any transition prior to self-motion. Then the first and eldest principle of motion, whether in things at rest or not at rest, will be the principle of self-motion; and that which is moved by others and can move others will be the second. 'True.' Let me ask another question: