We have discussed the fact that ideas or concepts occur to people, are the objects of various propositional attitudes, are parts of other ideas, are expressed by words, are acquired and possessed, represent things, and have contents or objects. Another characteristic property of ideas, one that lends itself to mathematical representation and experimental study, is their association with other ideas. We will observe that the association of ideas with sensory images and percepts is an important ingredient in the automatic use and understanding of language. We will also show that neural networks provide a good model and plausible basis for associative networks. But we will examine association mainly because it is a phenomenon distinctive of thought and ideation as opposed to belief and desire. Moreover, it is a causal relationship among ideas distinct from the structural relations obtaining among parts of the same thought. As such, the knowledge that ideas can be associated increases our understanding of the notion of an idea, as does recognition of the fact that ideational structure cannot be reduced to the relation of association.

Historically, philosophers and psychologists who gave ideas a central place in their theories tended to be associationists. Association psychology was the attempt to analyze and explain all mental phenomena in terms of the association of ideas. This resulted either in ignoring critically important mental phenomena or, more commonly, in applying the term “association” wherever two distinct elements were related in any way, which emptied the term “association” of all useful content.