The topic of concepts lies at the intersection of semantics and philosophy of mind. A concept is supposed to be a constituent of a thought (or ‘proposition’) rather in the way that a word is a constituent of a sentence that typically expresses a thought. Indeed, concepts are often thought to be the meanings of words (and will be designated by enclosing the words for them in brackets: [city] is expressed by ‘city’ and by ‘metropolis’). However, the two topics can diverge: non-linguistic animals may possess concepts, and standard linguistic meanings involve conventions in ways that concepts do not.
Concepts seem essential to ordinary and scientific psychological explanation, which would be undermined were it not possible for the same concept to occur in different thought episodes: someone could not even recall something unless the concepts they have now overlap the concepts they had earlier. If a disagreement between people is to be more than ‘merely verbal’, their words must express the same concepts. And if psychologists are to describe shared patterns of thought across people, they need to advert to shared concepts.
Concepts also seem essential to categorizing the world, for example, recognizing a cow and classifying it as a mammal. Concepts are also compositional: concepts can be combined to form a virtual infinitude of complex categories, in such a way that someone can understand a novel combination, for example, [smallest sub-atomic particle], by understanding its constituents.
Concepts, however, are not always studied as part of psychology. Some logicians and formal semanticists study the deductive relations among concepts and propositions in abstraction from any mind. Philosophers doing ‘philosophical analysis’ try to specify the conditions that make something the kind of thing it is – for example, what it is that makes an act good – an enterprise they take to consist in the analysis of concepts.
Given these diverse interests, there is considerable disagreement about what exactly a concept is. Psychologists tend to use ‘concept’ for internal representations, for example, images, stereotypes, words that may be the vehicles for thought in the mind or brain. Logicians and formal semanticists tend to use it for sets of real and possible objects, and functions defined over them; and philosophers of mind have variously proposed properties, ‘senses‘, inferential rules or discrimination abilities.
A related issue is what it is for someone to possess a concept. The ‘classical view’ presumed concepts had ‘definitions’ known by competent users. For example, grasping [bachelor] seemed to consist in grasping the definition, [adult, unmarried male]. However, if definitions are not to go on forever, there must be primitive concepts that are not defined but are grasped in some other way. Empiricism claimed that these definitions were provided by sensory conditions for a concept’s application. Thus, [material object] was defined in terms of certain possibilities of sensation.
The classical view suffers from the fact that few successful definitions have ever been provided. Wittgenstein suggested that concept possession need not consist in knowing a definition, but in appreciating the role of a concept in thought and practice. Moreover, he claimed, a concept need not apply to things by virtue of some closed set of features captured by a definition, but rather by virtue of ‘family resemblances’ among the things, a suggestion that has given rise in psychology to ‘prototype’ theories of concepts.
Most traditional approaches to possession conditions have been concerned with the internal states, especially the beliefs, of the conceptualizer. Quine raised a challenge for such an approach in his doctrine of ‘confirmation holism’, which stressed that a person’s beliefs are fixed by what they find plausible overall. Separating out any particular beliefs as defining a concept seemed to him arbitrary and in conflict with actual practice, where concepts seem shared by people with different beliefs. This led Quine himself to be sceptical about talk of concepts generally, denying that there was any principled way to distinguish ‘analytic’ claims that express definitional claims about a concept from ‘synthetic’ ones that express merely common beliefs about the things to which a concept applies.
However, recent philosophers suggest that people share concepts not by virtue of any internal facts, but by virtue of facts about their external (social) environment. For example, people arguably have the concept [water] by virtue of interacting in certain ways with H2O and deferring to experts in defining it. This work has given rise to a variety of externalist theories of concepts and semantics generally.
Many also think, however, that psychology could generalize about people’s minds independently of the external contexts they happen to inhabit, and so have proposed ‘two-factor theories’, according to which there is an internal component to a concept that may play a role in psychological explanation, as opposed to an external component that determines the application of the concept to the world.
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