              As for the necessity claim, Aristotle does not suppose that all phenomena admit of all four causes. Thus, for example, coincidences lack final causes, since they do not occur for the sake of anything; that is, after all,  what makes them coincidences.  If a debtor is on his way to the market to buy milk and she runs into her creditor, who is on his way to the same market to buy bread, then she may agree to pay the money owed immediately. Although resulting in a wanted outcome, their meeting was not for the sake of settling the debt; nor indeed was it for the sake of anything at all. It was a simple co-incidence. Hence, it lacks a final cause. Similarly, if we think that there are mathematical or geometrical abstractions, for instance a triangle existing as an object of thought independent of any material realization, then the triangle will trivially lack a material  cause.[17]   Still, these significant exceptions aside, Aristotle expects the vast majority of explanations to conform to his four-causal schema. In non-exceptional cases, a failure to specify all four of causes, is, he maintains, a failure in explanatory adequacy.