1.Material substances that are of the same species are numerically multiple.
2.This multiplicity must have its source in one of the general principles of material substances.
3.Since species is formal, this formality must then be multiplied either according to itself or according to something else.
4.If it is multiplied according to itself, then we would have distinct formalities by which something is made a member of a species.
5.To have distinct formalities, though, is to have different species. Different forms cannot constitute identical objects.
6.Therefore form cannot be the cause of numerical division in species.
7.The formality, then, must be multiplied according to something else.
8.We are left with matter, or the subject into which the substantial form is received.
9.Therefore matter is the principle by which members of the same species are individuated.