14. Why is Quality, again, not included among the Primaries? Because like Quantity it is a posterior, subsequent to Substance. Primary Substance must necessarily contain Quantity and Quality as its consequents; it cannot owe its subsistence to them, or require them for its completion: that would make it posterior to Quality and Quantity.
Now in the case of composite substances- those constituted from diverse elements- number and qualities provide a means of differentiation: the qualities may be detached from the common core around which they are found to group themselves. But in the primary genera there is no distinction to be drawn between simples and composites; the difference is between simples and those entities which complete not a particular substance but Substance as such. A particular substance may very well receive completion from Quality, for though it already has Substance before the accession of Quality, its particular character is external to Substance. But in Substance itself all the elements are substantial.
Nevertheless, we ventured to assert elsewhere that while the complements of Substance are only by analogy called qualities, yet accessions of external origin and subsequent to Substance are really qualities; that, further, the properties which inhere in substances are their activities [Acts], while those which are subsequent are merely modifications [or Passions]: we now affirm that the attributes of the particular substance are never complementary to Substance [as such]; an accession of Substance does not come to the substance of man qua man; he is, on the contrary, Substance in a higher degree before he arrives at differentiation, just as he is already "living being" before he passes into the rational species.