One might say that according to the principle of the compositionality of representations atomic representations make the same semantic contribution in every context in which they occur. See systematicity , productivity , symbolicism . The locus classicus for this argument is Fodor and Pylyshyn, 1988. According to Fodor and Pylyshyn, in normal cognitive agents, there exist intrinsic connections between some thoughts and others.
Thoughts come in clumps.
This putative fact is the systematicity of cognitive representations. The compositionality of representations says something about the nature of the thoughts that are intrinsically connected. The thoughts in the clumps have common terms and predicates, for example. Thus, the thoughts John loves Mary and Mary loves John are compositional sets of representations since they both represent John, loving, and Mary. According to Fodor and Pylyshyn, the reason that cognitive representations are compositional, as well as systematic is that there exists a syntactically and semantically combinatorial language of thought that respects the principle of compositionality, that atomic representations mean the same thing in all contexts in which they occur.