 An alternative conception of concepts takes concepts to be abstract objects of one type or another. The idea behind this view is that concepts are the meanings (or “contents”) of words and phrases as opposed to mental objects or mental states.  This type of view has most prominently been associated with the view that concepts are Fregean senses (e.g., Peacocke 1992, Zalta 2001), so it is this version of the view that concepts are abstract objects that we will focus on here. For proponents of this view, concepts, as meanings, mediate between thought and language, on the one hand, and referents, on the other. An expression without a referent (“Pegasus”) needn’t lack a meaning, since it still has a sense. Similarly, the same referent can be associated with different expressions (e.g., “Eric Blair” and “George Orwell”) because they convey different senses.  Senses are more discriminating than referents. Each sense has a unique perspective on its referent—a mode of presentation which represents the referent in a particular way. Differences in cognitive content trace back to differences in modes of presentation. It’s for this reason that the thought that George Orwell is Eric Blair lacks the triviality of the thought that George Orwell is George Orwell even though George Orwell and Eric Blair are the same person. Philosophers who take concepts to be senses particularly emphasize this feature of senses. Christopher Peacocke, for example, locates the subject matter of a theory of concepts as follows: “Concepts C and D are distinct if and only if there are two complete propositional contents that differ at most in that one contains C substituted in one or more places for D, and one of which is potentially informative while the other is not” (Peacocke 1992, p. 2). In other words, C and D embody differing modes of presentation. (See the entry  Frege  for discussion of the sense/reference distinction and for more on the explanatory functions associated with senses. To avoid terminological confusion, we should note that Frege himself did not use the term "concept" for senses, but rather for the referents of predicates. Similarly, it is worth noting that Frege uses the term "thought" to stand for propositions, so for Frege thoughts are not psychological states at all but rather the meanings of psychological states.)