… is from page 56 of Henry Hazlitt’s posthumously published 1997 volume, Is Politics Insoluble?  (original emphasis):
We should not take or judge an action in accordance with what we think would be its consequences considered as an isolated act. Not only can we never be certain what such consequences would be, but with such a moral code (or lack of code) we would never be able to depend on each other’s conduct, and we would fall far short of that social cooperation by which we most fully promote our own and each other’s ends. Moral action, for the most part, is action in accordance with accepted principles or rules. It is only when each of us can be depended upon to act consistently in accordance with such principles or rules that we can depend on each other. It is only when we can rely on each other to keep promises, to tell the truth, to refrain from theft, fraud, and violence, and to help each other in emergencies, that we can best promote that social cooperation so essential to attaining our individual ends.