Thus a crystal would not satisfy the condition of 'indivisibile in se' ( = in itself indivisible). However it satisfies the condition of 'indivisum in se' ( = by itself undivided), i.e. it is a genuine unity, and in this way the crystal is an individuum after all, according to the classical definition of "individuum" : indivisum in se, divisum ab alio ( = undivided in itself, devided from others ), and also according to St Thomas :
"It belongs to the concept of an individual that it is in itself undivided, and divided from others by an ultimate division."The single (in contrast to a twin) crystal is not an aggregate, but a Totality, and because of that undivided. The division from other crystals is not accomplished by an ultimate, last possible, division however. I.e. a crystal individual can still be further divided without loosing its specific identity (without changing into another species). Crystal individuals are not ultimate indivisible unities, i.e. they are not unities which, when further divided, change their specifity, like for instance chemical molecules, which do specifically change when divided. Chemical molecules (i.e. freely existing molecules, not received into the framework of a crystal) can indeed be considered as the final results of idem specie division, and as such they are divided (from each other) by an ultimate possible division.
(St Thomas, In Boëth. De Trin. q.4, a.2, ad 3).
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