In the opening pages of More Kinds of Being: A Further Study of Individuation, Identity, and the Logic of Sortal Terms (Blackwell, 2009), E. J. Lowe distinguishes five uses of ‘is’ as a copula: 1. The ‘is’ of attribution, as in ‘Socrates is wise’ and ‘Grass is green’.2. The ‘is’ of identity, as in ‘Napoleon is Bonaparte’ and ‘Water is H2O’.3. The ‘is’ of instantiation, as in ‘Mars is a planet’ and ‘A horse is a mammal’.4. The ‘is’ of constitution, as in ‘This ring is gold’ and ‘A human body is a collection of cells’.5. The ‘is’ of existence, as in ‘The Dodo is no more’.He says some may be reducible to others, and that one or two must be primitive. I thought this was a helpful spread.
That is indeed helpful, but here are some comments and questions.
1. First of all, I would be surprised if Lowe referred to the five uses as five uses of 'is' as a copula. The 'is' of existence is not a copula because it doesn't couple. There is no copulation, grammatical or logical, in 'God is.' The 'is' of existence does not pick out any sort of two-termed relation such as identity, instantiation, or constitution. Calling the 'is' of identity a copula is a bit of a stretch, and I don't think most philosophers would.
2. Is there a veritative use of 'is'? 'It is so.' 'It is the case that Frege died in 1925.' One could say, though it is not idiomatic: 'Obama's being president is.' One would be expressing that the state of affairs obtains or that the corresponding proposition is true. So it looks as if there is a veritative use of 'is.'
3. Reducibility of one use to another does not show that they are not distinct uses. Perhaps the veritative use can be reduced to what Lowe calls the attributive use. Attributions of truth, however, imply that truth is a property. Frege famously argued that truth cannot be a property. That is a messy separate can of worms.
4. There are also tensed and tenseless uses of 'is.' 'Obama is president' versus '7 + 5 is 12.' With respect to the latter, it would be a bad joke, one reminiscent of Yogi Berra, were I to ask,"You mean now?" Yogi Berra was once asked the time. He said,"You mean now?"
'Hume is an empiricist' can be used both in a tensed way and an untensed way. If I say that Hume is an empiricist what I say is true despite the present nonexistence of Hume. 'Grass is green,' however, is never used in a tensed way, though one can imagine circumstances in which it could.
5. One and the same tokening of 'is' can do more than one job. Is the 'is' in 'Max is black' as used by me in the presence of my cat Max the 'is' of predication merely? I don't think so. It also expresses existence. But this requires argument:
1. 'Max is black' and 'Black Max exists' are intertranslatable.
2. Intertranslatable sentences have the same sense.
3. 'Max is black' and 'Black Max exists' express the very same (Fregean) sense.
4. Both sentences express both predication and existence: a property is predicated of something that cannot have properties unless it exists.
5. The 'is' in 'Max is black' has a double function: it expresses both predication and existence.
Note that both sentences include a sign for the predicative tie. The sign is 'is' in the first sentence and in the second sentence the sign is the immediate concatenation of 'black' and 'Max' in that order. This shows that to refer to logical (as opposed to grammatical) copulation does not require a separate stand-alone sign. 'Black Max exists' expresses both existence via the sign 'exsts' and predication via the immeditae concatenation of 'black' and 'Max' in that order in the context of the sentence in question.