Individuation by acquaintance and by stipulation

Macbeth the hallucinator sees a dagger. There is no dagger there to be seen: no ordinary steel dagger before his eyes, no miniature dagger on his retina or inside his brain, no ghostly dagger of spook-stuff. There is no reason to think that our world contains any such thing. But the lack of a dagger makes it mysterious how we can describe Macbeth's state, as we do, by means of predicates applying to the dagger he seems to see - it is bloody, it has a wooden handle ' and not to the nerve signals, brain states, and other nondaggers that really exist. Notoriously, if we try to describe Macbeth adverbially - he is appeared to daggerishly, bloody-and-wooden-handled-daggerishly, and so on - it becomes plain that we only understand our Macbeth-descriptions by understanding the dagger-descriptions that are built into them. How so, if there is no dagger there to describe?