  One argument against postulating abstract objects is based on Ockham's razor. According to this principle one should not multiply entities or kinds of entities unnecessarily. Thus if one can show that certain concrete objects can perform the theoretical roles usually associated with abstract objects, one should refrain from postulating abstract objects. The effectiveness of this kind of appeal to Ockham's razor is, of course, conditional upon our having been shown that concrete objects can play the theoretical roles associated with abstract objects. But if every theoretical role played by abstracta can be played by concreta and vice versa, then one needs a further reason why one should postulate concreta only rather than abstracta only. Sometimes the only evidence for the existence of the abstracta in question is that they perform the theoretical role in question. In that case one can use the principle that one should not postulate ad hoc entities or kinds of entities unnecessarily (Rodriguez-Pereyra 2002, 210–16). That is, one should not postulate, if possible, entities for which there is no independent evidence, i.e. entities for the existence of which the only evidence available is that they satisfactorily perform a certain theoretical role.