In many languages it may be impossible to devise a procedure for determining which short environments over what limits should be set up as the differentiating ones for various substitution classes. If we select '—ing' as a diagnostic environment, we would get a class containing 'do, have, see', etc., but not 'certain'. If we select 'un—' as the environment, we obtain a class with 'do, certain', etc., but not 'have', and with see only if '–en' or '–ing' follow. We could obtain many different classifications of the same morphemes.
- Harris (1951), Pag. 256