The operation of substitution is basic in descriptive linguistics. Not only is it essential in phonemics, but it is also necessary for the initial setting up of morphemes, for the recognition of morpheme boundaries.
- Harris (1946), a pag.163

The procedure outlined here [position analysis] could be paralleled by a series of substitutions beginning with the whole utterance and working down, instead of beginning with single morphemes and working up. In that case we would have to find formal criteria for breaking the utterance down at successive stages. This is essentially the difficult problem of determining the immediate constituents of an utterance. It is not clear that there exists any general method for successively determining the immediate constituents, when we begin with a whole utterance and work down. In any case, it would appear that the formation of substitution classes presents fewer theoretical difficulties if we begin with morphemes and work up.
- Harris (1946), a pag.178