"Perhaps the biggest thing to remember here is that a set is a purely abstract concept, whereas an array is a concrete, tangible, explicit list. When we talk about sets, we’re reasoning in general about large conceptual things, whereas when we deal with arrays, we’re normally iterating through them for some specific purpose. You can’t iterate through a set very easily because (1) there’s no order to the members, and (2) there might well be infinitely many of them anyway."