When Aristotle defined the 10 Categories, he first made a broader classification between substance and accident. Substance was then named as the first Category, and the other 9 Categories were classified as Accidents. Any accident, whether quality, quantity, relation, action, or passion, only exists insofar as it inheres in a substance. For example, you might have a tall tree. Tallness, is a real quality/quantity, but it cannot exist outside of the tree. You do not walk down the street and see “tallness” walking by.
Least we think that because of this, only substances are real and accidents are not, we must also realize that substances don’t often exist without accidents, this is always true of material substances. Just as you cannot see tallness walking down the street, you cannot see a dog walking down the street that did not have some quality of color, for without color, nothing would be visible at all. Substance and accidents are related in a way to what Modern Philosophers (and I use that term in all of its negative connotation) call noumenal and phenomenal realities. Indeed many modern scientists argued that only the perceptible accidents were real, and that since we could not directly perceive substances, they must not be real.
Anyway, passions and actions are really real, but they also must inhere as accidents of some substance, i.e. some substance acts, or some substance receives.