"Just this once"
Suppose someone wants to lose weight
and is trying to decide whether to have one more spoonful of ice
cream. It's obvious that one more spoonful won't make any
difference to the overall goal of a healthy weight. So why not just
go ahead? Wouldn't it be irrational self-punishment not to? It's
not an answer to say that there will be a problem if he always has
an additional spoonful, because that's not the question being
considered. The issue is whether to have
this
spoonful.
The problem's the same as the
sorites
paradox: if one grain of wheat isn't a heap (Gk.
soros),
and two grains aren't either, and if a single additional grain
never makes something that isn't a heap into a heap (it's hard to
see how it could), then how could you ever get a heap of grain by
piling up one grain after another?
Another example is procrastination. If you don't feel like doing
something there's never a reason to do it just yet, you could
always wait until after you look at Drudge or whatever. So it seems
that to act reasonably -- in a way that makes sense overall --
always requires an element of irrationality in your particular
actions.