If it is true that...

Accidents are caused by the essential principles of a thing according to its perfect act or result from an aptitude in the substance

Then it must be true that...

Accidents are caused by either the form or matter of a substance

If it is true that...

Substantial form and matter do not have complete essences individually, but together they form a complete essence

and

Accidents are caused by the essential principles of a thing according to its perfect act or result from an aptitude in the substance

Then it must be true that...

In accidents, the genus, difference, and species are taken differently than in substances