What's in a Category? [Part 2]

Last week I wondered about the notion of syntactic category, aka part of speech (POS). My worry is that we have no clear idea what kind of work POS are supposed to do for syntax. We have some criteria for assigning POS to lexical items (LI) --- morphology, distribution, semantics --- but there are no clear-cut rules for how these are weighed against each other. Even worse, we have no idea why these are relevant criteria while plausible candidates such as phonological weight and arity seem to be irrelevant.¹ So what we have is an integral part of pretty much every syntactic formalism for which we cannot say

• what exactly it encompasses,
• why it is necessary,
• why it shows certain properties but not others.

Okay, that's a pretty unsatisfying state of affairs. Actually, things are even more unsatisfying once you look at the issue from a formal perspective. But the formal perspective also suggests a way out of this mess.

What's in a Category? [Part 1]

Norbert's most recent comments on Chomsky's lecture implicitly touched on an issue that I've been pondering ever since I realized how categories can be linked to constraints. Norbert's primary concern is the role of labels and how labeling may drive the syntactic machinery. Here's what caught my attention in his description of these ideas:

In effect, labels are how we create equivalence classes of expressions based on the basic atomic inventory. Another way of saying this is that Labeling maps a "complex" set {a,b} to either a or b, thereby putting it in the equivalence class of 'a' or 'b'. If Labels allow Select to apply to anything in the equivalence class of 'a' (and not just to 'a' alone), we can derive [structured linguistic objects] via Iteration.

Unless I'm vastly miscontruing Norbert's proposal, this is a generalization of the idea of labels as distribution classes. Linguists classify slept and killed Mary as VPs because they are interchangeable in all grammatical sentences of English. Now in Norbert's case the labels presumably aren't XPs but just lexical items, following standard ideas of Bare Phrase Structure. Let's ignore this complication for now (we'll come back to it later, I promise) and just focus on the issue that causes my panties to require some serious untwisting:

• Are syntactic categories tied to distribution classes in some way?
• If not, what is their contribution to the formalism?
• What does it mean for a lexical item to be, say, a verb rather than a noun?
• And why should we even care?