Galen’s causal accounts bear very clear echoes of Aristotle, both linguistically and in the fundamental approach to causal explanations in the natural world. At the same time, where Galen gives a listing of the number of different causes that may be used in such explanations, he refers explicitly to Plato and seems indebted to the Platonic tradition. In a passage of UP—the work which has the strongest Aristotelian echoes in this teleological sense—he gives a list of five causes, which seems closely related to one attested elsewhere in Platonist writing: final, efficient, material, instrumental, formal (UP VI.12; see Hankinson 1994b and 1998). On the other hand, the list comes in a dialectical context, and Galen is certainly not in general concerned to give accounts which mention all five. Of those in this “official” list, it is the teleological and material that recur and are of most significance elsewhere. One might ask about the role of the two other “Aristotelian” causes, the formal and the efficient. Explicitly, Galen has little role for the latter (although it is mentioned in an account of the demiurge’s construction of the body; see further below). As for the formal: certainly, forms (eidē) are of great importance in Galen’s biology, where an account of both form and matter is necessary in the description of animals and their development. Forms, indeed, have a close conceptual relationship with the teleological cause, as the way in which design or structure enters the physical world; at the same time, their relationship with matter and in particular with the concept of mixture is difficult to extricate (see further §6).