Seth posted a response to my 'global identifiers don't scale' post that I didn't expect. His point is that it is the meaning that isn't consistent across the semantic web, not the identifiers. I agree with him about meaning being inconsistent, but it's the distinction that confuses me - in my posts I've conflated identity with meaning, but Seth asserts that they are different things. He says:
"Take, for example, the URI <http://example.org/xyz>. You can say that: <http://example.org/xyz> a :Animal and I can say: <http://example.org/xyz> a :Car The context here is Who Said What. What's not up for question is that there is some Resource identified by <http://example.org/xyz>, and that when we use that identifier we are talking about the same thing."
Now this seems the wrong way round to me. For communication to be successful it must be grounded on common reality and understanding. That reality isn't the labels we use, but the underlying meanings associated with them. I.e. we attach labels to things in the real world, not the other way round.
In the end I don't think the distinction matters much: Regardless of which way you draw it, the important thing is the shared meaning associated with each label - that's how understanding is derived and knowledge transferred. The fact that we're using the same identifier string doesn't amount to anything useful if we're not denoting consistent meanings with it.