December’s letter of the month is to Paul Fitzgerald, a peer of David Lewis at Harvard University who wrote his thesis on the metaphysics of time under Donald C. Williams. This letter is in reaction to Fitzgerald’s The Truth About Tomorrow’s Sea Fight. 1969. Journal of Philosophy 66 (11):307-329.

In the first part of the letter Lewis offers a response on behalf of non-fullism – a theory of time according to which the future is not filled out with determinate content or it and its contents do not exist at all. His response implies that he has a strong preference for a fullist or eternalist theory of time, thus indicating to us when he had sympathies for a four-dimensionalist picture of time. His suggestion, on behalf of the non-fullist, is that there is a sharp cut-off between times – past and future – but that we do not know which space-times are simultaneous and so cannot know where the true border between past and future lies. In more contemporary terms, the idea is that a growing block theorist can posit some definite border between times but that she does not claim that we know what time is the present edge.

In the second part of the letter Lewis provides a novel explanation of truth-value gaps for someone – like Lewis – who adopts fullism or what we nowadays call eternalism. This novel explanation appeals to two important theories that Lewis had advanced by 1969: (1) counterpart theory plus the idea that possible worlds are of the same kind as our world, and (2) the indexical account of actuality. With these resources he shows how he can offer philosophical analyses that allow the eternalist to account for truth-value gaps (if such is needed to be explained or accounted for in one’s theory).

The last paragraph mentions Lewis’s conversations with Williams about the indexical account of actuality and sketches Lewis’s argument for the view, which appears in his well-known ‘Anselm and Actuality’ (1970), which Lewis had finished writing around the time of this letter. The last paragraph is interesting because he states the argument with a more overt connection to the temporal case than in ‘Anselm and Actuality’ (although the connection is of course there in the paper). The temporal case has come to be known as the ‘present problem’ or the ‘epistemic objection’ in the metaphysics of time. What it shows is that Lewis conceived of the argument as one that *assumes* modal realism and did not intend to put the argument forth as an *argument for* modal realism and specifically the theory that possible worlds are of the same kind as our world.

Let us consider the two cases. Take the temporal case first. Suppose all times are equally real. It is true that we cannot doubt that we are present. The indexical account of ‘present’ explains this fact, i.e., that we know that now is now. The indexical account helps expound eternalism by solving a problem about our knowledge of the present. Suppose we did not have such an explanation. Since all times are equally real, things in the past are just like things in the present. There’s nothing (it seems) that would ensure our knowledge that we know we are a present object and not some past (equally real) object. Now take the modal case. Analogously, it is assumed that there are possible worlds and that the contents of possible worlds are of the same kind as the contents of our world. Just like for all I know I might be a non-present object (assuming eternalism or the growing block theory), so too for all I know I might be a non-actual object (but only assuming modal realism). Of course, we know we are actual and we do not doubt this. But that’s the fact that is to be explained. If worlds were properties (ersatz worlds), just like if past times were properties (ersatz times), then the problem would not get off the ground (or at least, not according to the way Lewis was thinking of it). To sum up, the epistemic problem in the modal case is really designed to motivate the indexical account of actuality, which is used to solve the problem on the assumption that modal realism is true.