[I]t remains true that there will always be a very small chance of some totally unforeseen disaster resulting from your act. But it seems equally true that there will be a corresponding very small chance of your act resulting in something fantastically wonderful, although totally unforeseen. If there is indeed no reason to expect either, then the two possibilities will cancel each other out as we try to decide how to act.

Shelly Kagan, Normative Ethics, Boulder, 1998, p. 65