As the "generans" of terrestrial change moves what is light and heavy to another place by implanting a new form in a substance, so the magnet moves the iron by impressing upon it the quality by which it is moved. By virtue of the new quality, the iron is not in its natural place and moves accordingly. St. Thomas proved that the loadstone acts as a secondary "generans" in three ways: (1) the loadstone produces an effect not from any distance but only from a nearby position (showing that this motion is due to more than place alone), (2) rubbing the loadstone with garlic acts as if it impedes or alters the "virtus magnetis," and (3) the iron must be properly aligned with respect to the loadstone in order to be moved, especially if the loadstone is small. Thus the iron is moved by the magnet not only to a place, but also by changing and altering it: one has not only the change of locomotion but that of alteration. Moreover the source of this alteration in the iron is not the heavens but the loadstone. Accordingly the loadstone could cause change in another substance because it could influence the nature of the other substance.
[22] St. Thomas Aquinas, _op. cit._ (footnote 19), vol 7, _Scriptum in quartum librum sententiarum magistri Petri Lombardi_, lib. 4, disq. 33 (De diversis coniugii legibus), art. 1 (Utrum habere plures uxores sit contra legem naturae), p. 168. The same statement occurs in one of his most mature works, _op. cit._ vol. 20, _Summa theologica_, pars 3 (supplementum), quaestio 65 (De pluralitate uxorum in quinque articulos divisa), art. 1 (Utrum habere plures uxores sit contra legem naturae), p. 107.
[23] St. Thomas Aquinas, _op. cit._ (footnote 19), vol. 8, _Quaestio unica: de spiritualibus creaturis_, art. 2 (Utrum substantia spiritualis possit uniri corpori), p. 404. See also vol. 9, _Summa contra gentiles_, lib. 3, cap. 92 (Quomodo dicitur aliquis bene fortunatus, et quomodo adjuvatur homo ex superioribus causis), p. 344; and vol. 17, _Opuscula, De operationibus occultis naturae ad queindam militem ultramontem_, pp. 213-214.
[24] St. Thomas Aquinas, _op. cit._ (footnote 19), vol. 8, _Quaestio unica: de anima_, art. 1 (Utrum anima humana possit esse forma et hoc aliquid), p. 437. See also vol. 8, _Quaestio: De veritate_, quaestio 5 (De providentia), art. 10 (Utrum humani actus a divina providentia gubernentur mediis corporibus coelestibus), p. 678.