 Before we survey the various proposals for drawing the abstract/concrete distinction, we should briefly say why the distinction has been thought to matter. Among philosophers who take the distinction seriously, it is generally supposed that while concrete objects clearly exist, abstract entities are problematic in distinctive ways and deny the existence of abstract entities altogether. In this section we briefly survey the arguments for nominalism and the responses that platonists have offered. If the abstract objects are unified as a class, it is because they have some feature that generates what seems to be a distinctive problem—a problem that nominalists deem unsolvable and which platonists aim to solve. Before we ask what the unifying feature might be, it may therefore help to characterize the various problems it has been thought to generate.