In this paper I will examine a difficulty (aporia), which Aristotle raises in Physics B 5.197a20-25: whether fortuitous things can be held as causes of luck. I will show the meaning of the passage and its relevance within the context of Aristotelian extended analysis of the notions of ``luck'' and ``chance''. I will claim that, in facing the problem, Aristotle has a twofold aim. Firstly, he wants to explain why luck is, after having enquired whether it exists and what it (...) is. Secondly, he wants to prevent his pupils, among others, from denying causal efficacy to luck, by reducing fortuitous events, that are the result of accidental causes, to the action of per se causes, such as nature and human deliberation. (shrink)