But participation is, as it were, to take a part. And therefore when something receives particularly what belongs to another universally, it is said to participate in that, as main is said to participate in animal since man does not possess the intelligible content of animal according to its entire community (i.e. universality); and in the same way Socrates participates in man. Similarly, a subject participates in its accident and matter in form, since a substantial or accidental form, which is common from its intelligible content, is determined to this or that subject. Similarly, too, an effect is said to participate in its cause, and especially when the effect is not equal to the power of its cause, as for example if we say that air participates in the light of the sun, since it does not receive light with the brilliance it has in the sun.
— St. Thomas, Commantary on De Hebdomadibus, lect. 2