7.422. IDENTITY OF INTER-SEGMENTAL RELATION AMONG PHONEMES. [...] It is also convenient to have the relation among segment definitions within one phoneme identical with the relation in other phonemes. This requires that the segments be grouped into phonemes in such a way that several phonemes have correspondingly differing allophones (i.e. segment members) in corresponding environments. E.g. English [p, t, k] all occur in /s—V/, as in 'stone'; [pʰ, tʰ, kʰ] all occur in /#—V/ as in 'tone'. We could have grouped [p] and [tʰ] together, since they are complementary. But the above criterion directs us (barring other relevant relations) to group [p] with [pʰ] into /p/, and similarly for /t/, /k/. For if we do so, we can say that the /#—V/ member of all these phonemes is virtually identical with the /s—/ member except that [ʰ] is added; such a simple general statement would not have been possible if we had grouped the segments differently.
- Harris (1951), a pag.66