Reading Questions: Philoponus on Ballistic Motion
(Questions for reflection, which cannot necessarily be answered simply by reading the text but should be provoked by it, are marked with an asterisk.)
John Philoponus, Commentary on Aristotle's Physics, pp. 639.3-642.9 (Vitelli)
1. Who was John Philoponus? When, approximately, did he live? Where did he live?
2. What is antiperistasis? Can you think of an uncontroversial example of antiperistasis with common objects?
3. Does Aristotle's treatment of ballistic motion in Physics IV, 8 appear to be a considered theory or an off-the-cuff suggestion? (This is not the same question as whether his position on the matter is true or false.) What clues in the text indicate that your answer is correct?
4. What is a plenum? According to Aristotle, is the space in which objects move accidentally a plenum, or does it have to be a plenum? (You may want to re-read the whole of Physics IV, 6-8 here.)
5. When an object moves in a plenum of the sort envisaged by Aristotle, what must happen immediately behind the object? How might this consideration have influenced Aristotle's theory of antiperistasis as a principle for ballistic motion?
6. What does this answer show about the relation of Aristotle's theory of motion and his position regarding the plenum?
7. Philoponus is concerned with the direction that the air will be moving as the missile (a rock, ball, or arrow) travels through it. What, in particular, does he think one would be "hard put to it" to say?
*8. Evaluate Philoponus's argument at the point where he says that the antiperistasis explanation "borders rather on the fantastic." Has he brought any observations to bear? What should a reasonable non-specialist (without the benefit of modern physics) have believed, given only Aristotle's views and Philoponus's criticisms up to that point?
*9. In the next paragraph ("Again, the air that is in front..."), Philoponus argues that an explanation of an arrow's motion through antiperistasis would lead to a discontinuous motion of the arrow. Try to elaborate his argument here and make it compelling. What assumptions does he tacitly make? Do they seem reasonable?
*10. A few paragraphs down ("For it would be possible, without such contact, ..."), Philoponus proposes some experiments to test whether air is the main cause of projectile motion. Try to think of a counter-argument Aristotle might use in responding to these suggested experiments. (Would he disagree with Philoponus about the outcome of the experiments, or would he agree about their outcome but contend that they do not fairly test his theory?) Who do you think comes off better in the dispute at this point?
11. In the final paragraph, Philoponus makes a statement for which he is justly famous: "Rather is it necessary to assume that some incorporeal motive force is imparted by the projector to the projectile, ..." What does this mean? Try to express it more clearly without changing its meaning or making it more precise than it is.