For Aristotelian physics the membership of an object in a given class was of critical importance, because for Aristotle the class defined the essence or essential nature of the object, and thus determined its behavior in both positive and negative respects.
p. 143 Donald P. Spence (1994) The Rhetorical Voice of Psychoanalysis. p. 50 summarized this quote as "Class membership defined the essence or essential nature of the object". - The conflict between Aristotelian and Galileian modes of thought in contemporary psychology, 1931