DISTINCTION BETWEEN ESSENTIAL AND MODAL QUALITIES.
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.