It seems that it is self evident. After all, a set A consists of things. These things exist as part of the set. Therefore the idea is that I can think about the item of the set without really thinking about the other items in that set. The set is in a sense the totality of the parts. Why is this controversial then? Are the parts not prior to the whole? It would be quite weird if I think of an item in the set, that suddenly the item just disappears or become undefined. But then, it couldn't have been defined in the first place. If this is the case, then how would the set even exist? Basically, if I can't pick a item in the set in principle, then are no parts of the whole, which nullifies the idea of a set in the first place.