Two kinds of qualities must be distinguished; the essential quality,
which is a peculiarity of its being, and the mere quality, which
affects the being's classification. The mere quality introduces no
change in the essence, and causes none of its characteristics to
disappear; but, when the being exists already, and is complete,
this quality gives it a certain exterior disposition; and, whether
in the case of a soul or body, adds something to it. Thus visible
whiteness, which is of the very being of white lead, is not of the
being of the swan, because a swan may be of some color other than
white. Whiteness then completes the being of white lead, just as heat
completes the being of fire. If igneousness is said to be the being
of fire, whiteness is also the being of white lead. Nevertheless,
the igneousness of the visible fire is heat, which constitutes the
complement of its being; and whiteness plays the same part with respect
to white lead. Therefore (differing according to the difference of
various beings) the same things will be complements of being, and
will not be qualities, or they will not be complements of being, and
will be qualities; but it would not be reasonable to assert that
these qualities are different according to whether or not they are
complements of being, since their nature is the same.