It seems reasonable to believe that there might have existed things other than those that in fact exist, or have existed. But how should we understand such claims? Standard semantic theories exploit the Leibnizian metaphor of a set of all possible worlds: a proposition might or must be true if it is true in some or all possible worlds. The actualist, who believes that nothing exists except what actually exists, prefers to talk of possible states of the world, or of ways that a world might be. But even the actualist still faces the problem of explaining what we are talking about when we...