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.