In S.T.Ia 29. 1c. St Thomas says regarding the definition of PERSON, that the individual aspect is found to be in the genus of substance (in contrast with the other genera) in a special way, namely that the substance is individuated by itself (And then it would, after removal of the accidents, still be the same). Taken in it self this assertion would be equivalent to the nominalistic position of Ockham ((shortly) before 1300 -- 1349 or 1350), stating that every substance is individual (It is not -- still according to Nominalism -- individual by virtue of something other than the substance, because Quantity is, according to Ockham "transcendental", meaning that it is only distinguished from substance by the intellect). However we by now know what "substance is individuated by itself" means : It is individuated by its matter (but (matter) is influenced by quantity).
Of course this is not the case with repect to the "rational substances" (for instance the Divine Persons (of the Trinity)). The Divine Substances are individuals in virtue of themselves, because their form is ONE and SEPARATED (and they consist of form only). For the Divine Substances the only possibility of diversity (including numerical diversity) a difference in content of the form.
In the case of 'natural substances' diversity (numerical diversity) can also occur if a same form is being received in (numerical) other matters.
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