- What do we see when we see what isn't there? 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 non-daggers 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 under- standing the dagger-descriptions that are built into them. How so, if there is no dagger there to describe?
- The case of the missing dagger has been solved by inspector Hintikka. I accept his solution, differing only on points of detail, and I shall begin this paper by restating it in my own way.
- When Macbeth is appeared to daggerishly, his experience has informational content, and part of that content is that there is a dagger before him. His experience tends to modify his belief, and if he is fooled by his hallucination, then also part of the content of his belief is that there is a dagger before him. An ordinary dagger, not a spooky one; and before the eyes, not inside the head.
- The dagger Macbeth seems to see has the same status as Sherlock Holmes; or as the planet Vulcan, mistakenly posited to explain the perturbations of Mercury. The dagger, or Holmes, or Vulcan, exists according to something with false informational content, but does not actually exist.
- Informational content can be explained in terms of possibilities. The information admits some possibilities and excludes others. Its content is given by the division of possibilities into the admitted ones and the excluded ones. The information is that some one of these possibilities is realized, not any one of those.
Originally, Philosophical Review, Vol. 92, No. 1 (Jan., 1983), pp. 3-32
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