Thomas Metcalf

What does it mean to say that subject S is capable of task T?

I think we can say with some confidence that if S is capable of T, there's a possible world in which S performs T. But I'm not so sure about a conditional in the other direction. That is, we can't say that if there is a possible world in which S performs T, then S is capable of T.

It makes sense to say I'm incapable of flying under my own power. But there is a possible world in which I develop large, feathery wings that allow me to fly under my own power. So merely doing some task in a possible world doesn't indicate that I'm capable of that task. And I don't think I'd cease to exist if I sprouted these large, feathery wings.

Maybe it's true that if there's a possible world in which S performs T and possesses all of S's essential properties, then S is capable of T. But is this just an ad hoc addition to cover weird results in other possible worlds? And it still doesn't seem to solve the problem of flight under my own power, because it doesn't seem to be an essential property of me that I can't fly under my own power.

Maybe we need something like the following. If there's a possible world in which S performs T and S matches the definition given of S, then S is capable of T. But again, I worry that this might be ad hoc. Maybe we need to define T such that S has not gained some property that is required for her to perform T? But then I don't have the capability to buy a car. Maybe that's the result we want.

I tend to conclude that if S is capable of T, there's a possible world in which S performs T, and if there's a possible world in which S has not gained any property P necessary to perform T and yet performs T, then S is capable of T. I guess my only worry is the possibility of circularity when we're trying to figure out whether S has gained a property P -- how do we figure out whether P is such that it's necessary to perform T?

mattbballman

tom,

That seems like an appropriate analysis. To say that S performs T if and only if S performs T is surely circular. And if we say that S can do anything that S does then that would be ad hoc.

So, Freddoso and Flint are correct to incorporate the notion of a person's history. So we can say that someone S can perform actions A1 - An if and only if a person shares the same world-history as S and is logically capable of A1 - An. Whether this is sufficient or not is a matter hotly disputed by philosophers even today.

matt

Theli

Well... in this world you haven't developed wings yet, so you are incapable of flying until that little miracle takes place.
Personally, I don't see a problem here. It looks like another miss by modal logic.
If you really are capable of flying, then why don't you sprout wings and fly?
If are able to obtain P by choice at any time (wich I doubt), then you can gain the ability to fly, but you don't have that ability until you do obtain P.