We may now say that, in the intelligible world, qualities are the
characteristic differences in being or essence. These differences
effect distinction between the beings; in short, they cause them to
be beings. This definition seems reasonable. But it does not suit the
qualities below (in the sense-world); some are differences of being,
as biped, or quadruped (as thought Aristotle);[330] others are not
differences, and on that very account are called qualities. Still,
the same thing may appear a difference when it is a complement of
the being, and again it may not seem a difference when it is not a
complement of the being, but an accident: as, for instance, whiteness
is a complement of being in a swan, or in white lead; but in a human
being like you, it is only an accident (as thought Aristotle).[331] So
long as the whiteness is in the ("seminal) reason," it is a complement
of being, and not a quality; if it be on the surface of a being, it is
a quality.