Behind our ability to substitute parts of one utterance for parts of another lies the empirical fact that in every language the speakers recognize not an indefinitely large number of distinct, unsubstitutable sounds (so that every new utterance may contain a new distinct sound), but a relatively small stock of distinct classes of sounds. This stock is in general closed. I.e. when a sound occurs in speech we can in general assign it to one of relatively few classes of sounds used in describing the language; or we may say that the speaker and hearer react to it as to a member of one of these few classes of sounds in the language—or else as to a sound from outside their language. The classes of sounds recognized in the language are thus limited in number.
- Harris (1951), a pag.31, n.4