Assuming that there is no afterlife -- that you lose the ability to think or feel anything once your body dies -- is it irrational to fear death? Asked another way: Was Larkin wrong when he described the philosopher's contention that "no rational being can fear a thing it will not feel" as "specious stuff"?

The reasoning that in the absence of an afterlife it would be irrational to fear death dates back at least to the ancient Greek philosopher Epicurus, who wrote: "Accustom yourself to the belief that death is nothing to us. For all good and evil lie in sensation, whereas death is the absence of sensation… Therefore, that most frightful of evils, death, is nothing to us, seeing that when we exist death is not present, and when death is present we do not exist." There is a good collection of essays edited by John Martin Fischer called The Metaphysics of Death on this subject. Of particular interest to you might be the essay "Death" by Tom Nagel. Nagel argues (along similar lines to those used by Peter Lipton above) that there are things that can harm us (and thus that it would be rational to fear) that occur outside of our experience. Thus the mere fact that death is outside of our experience does not mean that it would be irrational to fear it.