A businessman contemplates buying a certain piece of property. He considers the outcome of the next presidential election relevant to the attractiveness of the purchase. So, to clarify the matter for himself, he asks whether he would buy if he knew that the Republican candidate were going to win, and decides that he would do so. Similarly, he considers whether he would buy if he knew that the Democratic candidate were going to win, and again finds that he would do so. Seeing that he would buy in either event, he decides that he should buy, even though he does not know which event obtains, or will obtain, as we would ordinarily say. It is all too seldom that a decision can be arrived at on the basis of the principle used by this businessman, but, except possibly for the assumption of simple ordering, I know of no other extralogical principle governing decisions that finds such ready acceptance.
Having suggested what I shall tentatively call the sure-thing principle, let me give it relatively formal statement thus: If the person would not prefer f to g, either knowing that the event B obtained, or knowing that the event ~B obtained, then he does not prefer f to g. Moreover (provided he does not regard B as virtually impossible) if he would definitely prefer g to f, knowing that B obtained, and, if he would not prefer f to g, knowing that B did not obtain, then he definitely prefers g to f.