In this talk I want to address what I believe is a serious challenge for Aristotle’s theory of causality, which he develops in the second book of the Physics. One of the more basic assumptions that Aristotle makes about nature is that the latter is a self-sufficient totality, which is intelligible in itself. Nature, in other words, is an internal origin or principle of motion (and rest) for those things that exist by nature. To give a causal explanation of the processes of nature should therefore amount to something like explicating the internal, and that is to say, natural features of the things that exist by nature. For this reason, natural generation must be carefully distinguished from artificial production, which comes about by the aid of external causes. Nature is not an artifact, and so the relation between cause and effect in natural generation must not be conceived of in terms of production. But if this is granted, then the notion of efficient causality, where the cause is thought to precisely bring about an effect to which it is externally related, should be highly problematic for Aristotle. Still, Aristotle is commonly believed to have introduced such a notion of cause in his natural philosophy. In my talk I shall show how Aristotle tries to interpret external causation in such a way that makes it possible to allow for this kind of causation within nature, without having to sacrifice the autonomy of nature.