15. How then do the four genera complete Substance without qualifying it or even particularizing it?
It has been observed that Being is primary, and it is clear that none of the four – Motion, Stability, Difference, Identity – is distinct from it. That this Motion does not produce Quality is doubtless also clear, but a word or two will make it clearer still.
If Motion is the Act of Substance, and Being and the Primaries in general are its Act, then Motion is not an accidental attribute: as the Act of what is necessarily actual (what necessarily involves Act), it is no longer to be considered as the complement of Substance but as Substance itself. For this reason, then, it has not been assigned to a posterior class, or referred to Quality, but has been made contemporary with Being.
The truth is not that Being first is and then takes Motion, first is and then acquires Stability: neither Stability nor Motion is a mere modification of Being. Similarly, Identity and Difference are not later additions: Being did not grow into plurality; its very unity was a plurality; but plurality implies Difference, and unity-in-plurality involves Identity.
Substance (Real Being) requires no more than these five constituents; but when we have to turn to the lower sphere, we find other principles giving rise no longer to Substance (as such) but to quantitative Substance and qualitative: these other principles may be regarded as genera but not primary genera.