What role did the formal cause play in Aristotle’s explanations?

In Aristotelianism, the formal cause serves as one of four fundamental types of explanation for why things are the way they are. It represents the essence, pattern, or structure that a fully realized object embodies.12

Rather than merely describing physical composition, the formal cause defines what a thing is. For instance, Aristotle noted that the specific proportion of string lengths in a lyre acts as the formal cause for one musical note being the octave of another.2

In the study of living beings, the formal cause is particularly significant as the "substantial form"—the organization of an organism that dictates its life cycle and characteristic activities. In these biological contexts, the formal cause often coincides with the final cause, as the development of an organism’s parts is understood as the actualization of a structure necessary to perform specific biological functions. By identifying this internal blueprint, Aristotle provided a framework to explain how entities maintain their identity and purpose throughout their existence.2