In Part 1 of this series, we introduced Aquinas’ argument from motion and provided some textual exegesis. Given the aforementioned exegesis and explanation, Aquinas’ argument from change can be formalized as follows:
P1: Some things change (i.e. there are changes).
P2: But change is the reduction of potential to actual (i.e. the actualization of potential).
P3 (P1, P2): Therefore, some things reduce from potential to actual.
P4: Whatever reduces from potential to actual is actualized by some other actually existent thing.
P5 (P3, P4): Therefore, some things are actualized by some other actually existent things.
P6: If some things are actualized by some other actually existent things, then there are chains of changes (i.e. one thing changed by another, in turn changed by another, and so on).
P7 (P5, P6): Therefore, there are chains of changes.
P8: If chains of changes were infinitely long, then there would be no first member in the series of changes.
P9: But if there were no first member in the series of changes, there would be no subsequent changes.
P10: But if some things change, then there are subsequent changes.
P11 (P1, P10): Therefore, there are subsequent changes.
P12 (P8, P9, P11): Therefore, it is not the case that chains of changes are infinitely long.
P13: If it is not the case that chains of changes are infinitely long, then such chains terminate in one first member (the unchanged changer or unactualized actualizer).
P14: If such chains terminate in one first member (the unchanged changer or unactualized actualizer), then God exists.
P15 (P12, P13, P14): Therefore, God exists.
Although Aquinas did not, of course, make every such step explicit in his presentation of the argument, all are either implicit in what he writes or are relatively unobjectionable fillers that are necessary for the argument’s validity. The formalized version of Aquinas’ argument, as stated above, is valid. P3 follows by substitution, with the premises in parenthesis as the sub-premises. P5 follows by universal instantiation. P7 and P11 follow by modus ponens. P12 follows by transitivity and modus tollens, and P15 follows by transitivity and modus ponens. It remains to be seen, however, whether the argument is sound. It is to this question that I shall turn in the next part of the series.