Central to Aristotle's metaphysics and epistemology is the claim that ‘aitia’ – ‘cause’ – is “said in many ways”, i.e., multivocal. Though the importance of the four causes in Aristotle's system cannot be overstated, the nature of his pluralism about aitiai has not been addressed. It is not at all obvious how these modes of causation are related to one another, or why they all deserve a common term. Nor is it clear, in particular, whether the causes are related to one another as species under a single genus, such that there is a univocal definition of ‘aitia’ which applies to all of them, or whether Aristotle means to assert that the four causes are homonyms. It is argued here that although there are strong reasons to group the four causes together, there are also powerful considerations on the side of homonymy. It is further argued that the four causes are more closely tied to the ontological theory of categories and predication than is often recognized. As a result, we can reconcile the competing demands of unity and plurality by taking one mode of causation, the formal cause, as basic, and accounting for the other modes with reference to it, in the manner of so-called pros hen homonyms.