I've been thinking today about how something's being epistemically rational doesn't match up with that thing's being, epistemically speaking, the best thing (or one of a number of optimal things) to do overall.
Suppose you think that withholding judgement as to whether or not the physical world exists would be crippling, in almost all ways, including epistemically. People who withhold judgement about whether there is a physical world are making a big mistake, insofar as they want to do well epistemically - they are pursuing a course of action which is epistemically hopeless. In order to do well epistemically, believing in the external world is the thing to do. It enables us to get on with all sorts of other important epistemic projects. Many of these are, of course, projects which depend on our assuming that the physical world exists (e.g. projects involving visual inspection). But I want to ignore those for now (for reasons that should become clear below). Let's focus instead on the projects which don't actually depend on that assumption, but are just such that we wouldn't ever get round to them if we were too busy worrying about whether the physical world existed or not.
If it's right that withholding judgement about the physical world would prevent our undertaking these projects, then believing in the physical world is, epistemically speaking, a good thing to do, insofar as it enables us to get on with all these projects.
But there could be a drawback: suppose belief in the external world is in fact not supported by evidence (or suppose that it has some other status you would normally take to suffice for irrationality). Then, epistemically speaking, believing in the external world has drawbacks as well as advantages - it enables us to get on with those other epistemic projects, but it requires us to do something that we would normally regard as epistemically dodgy. (NB: this would also mean that any projects which depended on the assumption that there is an external world could not really be counted as epistemic bonuses for the believer - they would share the epistemic dodginess of the assumption on which they depended.)
But suppose that we were eventually going to stop believing in the physical world, but only after we've got around to pursuing a good number of these other (independent) projects. Then, it seems, we would eventually end up in an epistemic situation which had the advantages of belief in the physical world but not the disadvantages.
Perhaps this would mean that believing in the external world was, epistemically speaking, the best thing to do overall. Imagine, furthermore, that it's even known by the subject to be epistemically speaking the best thing to do overall. Could we infer that it was epistemically rational?
That would seem very strange to me. I'd be interested to hear how others' intuitions go on this case, but mine is that the epistemic rationality of a belief in p at a time t is to be evaluated just with regard to the subject's epistemic state at t with regard to p. Considerations about one's future epistemic state with regard to other propostions shouldn't enter into it.