underlying phoneme

You miss the point that the orthography is a theoretical substrate for the phonology. We need an orthographic representation of the abstract phoneme which reflects the segmental syntax. Since the orthographic symbol "s" remains syntactically unchanged, that is why the symbol phoneme /s/ is utilized as "underlying" rather than symbol phoneme /z/.

I don't agree with your assertion that the orthography acts as a theoretical substrate for the phonology. Linguistics tries to avoid the orthography of a language. This is precisely why the International Phonetic Alphabet was developed.

But we'll indulge your contention. Orthographically speaking, if the plural were to be written as "z," it would remain syntactically--and also semantically--unchanged as well. So, what's to say "z" shouldn't be underlying?

(Sorry, earlier reply lost)

Only the history of "grammar" of the language decides that.

Phonology is to phonetics as economic theory is to accountancy. It is an abstraction. Some sort of orthographic representation is required to reflect the concept of sequential morphemes and traditional orthography is historically functional in that respect. The IPA has nothing to do with phonology. It is a phonetic convention.

Your assertion would limit phonology to languages that have an orthography. Many languages do not, or had not had an orthography, such as the Native American languages.

The history of grammar of the English language has shown that both /s/ and /z/ are allophones of the morpheme for plural. English orthography cannot decide which is the underlying phoneme because the plural morphemes /s/ and /z/ existed in the language long before it came to be written.

No. It would not. Unless you understand that phonology is abstract and representation is often historical we are talking past each other.

I realized since your first response when you did not address my question concerning phonology, but instead went off on a tangent about representation that we were talking past each other.

Then let's do try to focus, Gents, for this is very interesting.