Abstract: Examines how we should assess the effects of our acts, especially when we act together with other people, why we should reject the share-of-the-total view and accept the marginalist view, which appeals to the difference made by each act, why we should not ignore either small chances, or effects that are trivial or imperceptible. It also presents several cases in which effects are overdetermined. Rational altruism is also discussed.

Keywords: altruism, benefit, collective, harm, overdetermination, probability, rationality, risk,