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Peter King cites Duns Scotus' presentation of the Modal Argument against self change (additions by him):

[The Modal Argument] is taken from Phys . 3.2 [202 a 10–13], where Aristotle says “the mover moves insofar as it is in act, and the mobile is moved insofar as it is in potency, as is evident from the definition of motion given in [ Phys . 3.1 201 a 11–12]. 11 However, it is impossible that the same thing be at once in potency and in act with respect to the same and according to the same. Therefore, [nothing can be moved by itself].

And then goes on to give the following restatement of it:

The Modal Argument may be reformulated at a more general level as follows:

  • [A1] The subject of a change must be in potency to φ . (Definition of change)
  • [A2] Causes must “contain” their effects. (Causal Axiom 1)
  • [A3] Hence the cause of a change must be in act with respect to φ . (From [A2] and the definition of change)
  • [A4] Proximate causes must be spatio-temporally concurrent with their immediate effects. (Causal Axiom 2)
  • [A5] It is impossible for one and the same thing to be at once in potency and act with respect to the same and according to the same. (Application of the Law of Non-Contradiction to potency and act)

Therefore: Anything that changes must be changed by another.

Duns Scotus rejects the argument because of A5 and A3.

The question now is not if Duns Scotus is correct, but rather why he is not very obviously correct – how A5 and A3 could even be somewhat plausible rather than trivially wrong.

If we take, for example, a match and a matchbox and strike the match against the side of the matchbox, fire is produced. But why should we assume that the “form of fire” (if we grant the existence of such a form) is in actuality somewhere “on” the matchbox (or anywhere else)?

Can somebody give some explanation why one should not immediately discard A5 and A3? How could one regard them as reasonable?

  • Were you not satisfied by what I quoted in my answer to your question "How are 'causal' loops avoided in Aristotelian-Thomistic metaphysics?"? It seems you're asking about the "quidquid movetur ab alio movetur" principle again. – Geremia Jul 9 '17 at 2:10
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    But why should we assume that the “form of fire” (if we grant the existence of such a form) is in actuality somewhere “on” the matchbox (or anywhere else) strikes me as confusing Platonic forms with Aristotle's essences... – virmaior Jul 9 '17 at 6:03
  • @Geremia no, I found your answer impenetrable. If I had been satisfied by your answer, I would have accepted it. And also, the two questions are not remotely the same. It's not about causal loops this time, but about the axiom “the cause of a change must be in act with respect to φ” which one needs to exclude self change. – wolf-revo-cats Jul 9 '17 at 6:57
  • @Geremia so either you say something about A3 and A5, or you stay out of this discussion. It doesn't help to claim “you've asked this question before!” when those two questions are clearly different. – wolf-revo-cats Jul 9 '17 at 7:05
  • @virmaior you should go into a bit more detail. What's exactly the problem with this sentence? It can't be “form of fire” itself, that's something Aquinas and Aristotle mentioned. – wolf-revo-cats Jul 9 '17 at 7:14
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In Duns Scotus' philosophy some forms are active principles and have the power to produce an effect. There are two kind of powers qua efficient causes, natura and voluntas, But he distinguishes between the aptitudo and the excercitio of those powers. Talking about natural efficient powers, a form can have an apptitude but no excecises it because theres no subjet in potency near to activate her. So a form can be in actuallity on a subject by way of apptitude.

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