I am trying to understand the following passages from The Sound Pattern of English, by Noam Chomsky and Morris Halle.
Convention 1: : Every segment of a lexical matrix is automatically marked [+n] for every rule n. [...]
Convention 2 : Every segment of a lexical matrix μ is marked [αK] for each category [αK] to which μ belongs.
Thus, in particular, if a formative belongs to the lexical category [-n], each of its segments will be marked [-n] by Convention 2, after having automatically been marked [+n] by Convention 1. Thus every time a formative is an exception to a rule, there is a certain "cost" associated with this fact, namely, a certain category assigment must be given the lexical entry. But an item that does undergo a rule need not be specially marked. Thus only exceptions to a rule contribute to the complexity of the grammar in this connection.
(page 173—it's page 93 of the PDF I linked to)
Alongside of the partially systematic class of exceptions to rule (6), we also find purely idiosyncratic exceptions. For example, consider the rule that makes vowels non-tense before certain affixes (e.g. compare serene and serenity, obscene and obscenity). There are exceptions to this rule (e.g. obese-obesity, in most dialects) which simply must be categorized as such in the lexicon, these lexical features becoming segmental features by Convention 2. Each such example contributes to the complexity of the grammar, but there is obviously no question of rejecting the rule. Doing so would amount to treating each item as an exception, in the manner of item-and-arrangement grammars (see Chapter Three, Section 16), and there is surely no point to such a decision.
(page 174/page 94 in the PDF)
Chapter 3 section 16 compares such situations to regular and irregular verbs; irregular verbs are exceptions, but the fact that exceptions exist doesn’t negate the rules that apply to regular cases. It outlines three ways of dealing with phonological irregularities: 1) abandoning general rules and lexically specifying the irregular feature for each particular item; 2) classifying words as (+) regular and (-) regular; 3) reinterpreting the underlying form to make the rule apply (page 147/page 80 in the PDF). (This last option is the one Chomsky and Halle prefer; this discussion is a leadup to their analysis of forms like “ellipse” as having underlying segmental “e” at the end.)
Anyway, I guess I don’t understand the motivation for the analysis made in the quoted material. In what sense should we consider words like “obesity” an exception? Obviously, it is one in terms of frequency (there are more items in the lexicon that alternate than ones that don’t). But Chomsky and Halle seem to be saying that it is mentally classified as an exception by native speakers. If this is the case, I don’t understand why this extra complexity in the lexical representation of the word originated historically (the word used to be pronounced with /ɛ/, and over time came to be pronounced with /i/). My default intuition would be to consider the words with historical phonological alternations as marked, in the sense that they're harder to process psychologically (for example, I have to consciously think to apply them sometimes, and avoid saying stuff like /səˈriːnɪti/), and the non-alternating forms like "obesity" as unmarked. I would not reject the rule entirely, but I don't understand why the rule is taken to be the default situation.
Is Chomsky and Halle's viewpoint current in phonology, or do most people now have a different interpretation? I know there are other approaches to phonology, such as Optimality Theory, but I am only an amateur and not familar with them in any depth.