I know very little about the life of American linguist Robert Lees but he dropped two bangers in the early 1960s: his 1960 book on English nominalizations is heavily cited, and his 1961 phonology of Turkish has a lot of great ideas. In this passage, he seems to presage (though not formally) the idea that underspecified segments do not form singleton natural classes, and he correctly note that that’s a feature, not a bug.
The rest of the details of vowel- and consonant-harmony we shall discuss later; but there is one unavoidable theoretical issue to be settled in connection with this contrast between borrowed and native lexicon. The solution we have proposed for Turkish vowels, namely that they be written “archiphonemically” in all contexts where gravity is predictable by the usual progressive assimilation rules of harmony but be split into grave/acute pairs of “phonemes” elsewhere, will have the following important theoretical consequence. The phonetic rule which ensures the insertion of a “plus” or a “minus” sense for the gravity feature under harmonic assimilation must distinguish between occurrences of columns of features in which gravity is unspecified, as in the case of the “morphophoneme” /E/, from occurrences of the otherwise identical columns of features in the utterance being generated in which gravity has already been specified in a lexical rule, as in the corresponding cases of the “phonemes” /e/ and /a/, for now both the morphophoneme /E/ and the phonemes /e/ and /a/ occur simultaneously in the transcriptions. The gravity rule is intended to apply to /E/, not to /a/ or /e/. But this is tantamount to entering, as it were, a “zero” into the feature table of /E/ to distinguish it from the columns for /a/ and /e/, in which the feature of gravity has already been determined, or perhaps from other columns in which there is simply no relevant indication of this feature.Thus the system of phonetic decisions will have been rendered trinary rather than the customary binary. The objection to this result is not based on a predilection for binary features, though there are good reasons to prefer a binary system. Rather, it arises because in a system of phonological decisions in which rules may distinguish between columns of binary features differing solely in the presence of absence of a zero for some feature one may also ipso facto always introduce vacuous reductions or simplifications without any empirical knowledge of the phonetic facts.As a brief illustration of such an empty simplification, we might note that if a rule be permitted in English phonology which distinguishes between the features of /p/ and /b/ on one hand and on the other, the set of these same features with the exception that voice is unspecified (a set which we shall designate by means of the “archiphonemic” symbol /B/), then we could easily eliminate from English phonology, without knowing anything about English pronunciation, the otherwise relevant feature of Voice from all occurrences of either /p/ or /b/, or in fact from all occurrences of any voiced stop. Clearly, if a rule could distinguish /B/ from /p/ by the presence of zero in the voice-feature position, then that feature can be restored to occurrences of /b/ automatically and is thus rendered redundant. The same could then be done for inumerable [sic] other features with no empirical justification required.Thus, we must assume that any rule which applies to a column of features like /B/ also at the same time applies to every other type of column which contains that same combination of features, such as /p/ and /b/. This is tantamount to imposing the constraint on phonological features that they never be required to identify unspecified, or zero, features. To the best of our present knowledge, there seems to be no other reasonable way to prevent the awkward consequences mentioned above.To return to Turkish this decision means that the grammar is incapable of distinguishing native vowel-harmonic morphemes from borrowed non-vowel-harmonic morphemes simply be the presence of the archiphoneme /E/ in the former versus /e/ or /a/ in the latter. (Lees 1961: 12-14)
References
Lees, R. B. 1961. The Phonology of Modern Standard Turkish. Indiana University Press.