The nature of concepts, and their relation to the things 'of which they are the concepts', and to the minds which use or contemplate them, are among the most hotly disputed subject in philosophy. The present definition is not intended to prejudge or settle any of these issues, even if limitations of space make it appear to do so.
B. Defined as an aspect of thought, a concept is a kind of unit in terms of which one thinks; a unit smaller than a judgement, proposition, or theory, but one which necessarily enters into these. In an assertion, something is predicated of a concept, and the predicate itself can generally be re-described as a concept. At the same time, however, the concept is by no means an ultimate or indivisible unit, for concepts can be augmented or diminished by addition or subtraction of some feature. (For instance, one may say that someone's concept of social class does, or fails to, include the notion of differences in material rewards.) Moreover, while concepts occur within assertions or theories and are thus distinct from them, a proposition or theory or thesis as a whole can in turn be referred to as a further concept. For instance, R.Firth writes that 'some of Dr. Leach's concepts are of a special order…I refer to his thesis that seeking for power is the basis of social choice' (Foreword to E.R. Leach, Political Systems of Highland Burma, London: G. Bell, 1954, p. vii).
C. Concepts correspond to or 'are the meaning of' all meaningful words, with certain qualifications: (a) only one concept corresponds to two or more words with the same meaning; (b) there is a tendency to speak of concepts only with regard to words which do, or at least can, refer either to something that can exist or be imagined or to an operation that can be performed, and not in connection with words whose role is grammatical rather than designative (for instance, one may speak of the concept of sovereignty, of infinity, of addition, but not of the concept of 'and' - though one must add that the drawing of the lines beetween these kinds of meaning is difficult, unsettled, and controversial part of philosophy: (c) there is a tendency to speak of concepts in connection with general rather than singular terms (one is unlikely to speak of a 'concpet of John' or of a 'concpet of London'; in those cases the term 'conception' is more likely to be used. There are, however, exceptions, e.g., 'the concept of God').
The fact that concepts may be seen as the meanings of terms should not lead one to suppose that concepts are in some narrow sense linguistic entities: although concepts may be defined in terms of the rules governing the use of the words said to designate them, those rules determine (a) what things in the world are classed together (as 'falling' under the same concept'), (b) what features are grouped together (as 'being various characteristics of the same thing'), (c) what operations of measurement, classification, discrimination, etc., are performed by the man 'using the concept', and so on.
D. Discussions of concepts in the social sciences tend to be a matter of the choice of terms and, more importantly, of their definitions. One may talk both of discovering and of inventing concepts; also of changing and developing concepts. In as far as given theories require certain concepts, and in as far as concepts can be said to incorporate theories, there is no sharp line between choice of theories and choice of concepts. Nevertheless, whole theories are thought of primarily as true or false, concepts are more naturally described as applicable or inapplicable, valid or invalid, useful or useless.
Taken from A Dictionary of the Social Sciences eds. J. Gould and W. Kolb, Free Press, 1964.