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?