Key Arguments

• Parfit formulates the mistake explicitly: '(The Second Mistake) If some act is right or wrong because of its effects, the only relevant effects are the effects of this particular act.'
• He immediately claims 'This assumption is mistaken in at least two kinds of case,' signalling that there are systematic counterexamples.
• In overdetermination cases like Case One, if we look only at the effects of each individual act, then given what the other agent does, each agent’s act 'would have made no difference'; this pushes us, if we accept the Second Mistake, towards the absurd conclusion that two simultaneous killers 'do not act wrongly.'
• In coordination and public‑good problems, if we look only at the effects of each single act, we can say that each person has done what has the best consequences 'given what the others did', and yet jointly they fail to produce the best outcome—showing that single‑act focus is inadequate.

Source Quotes

26. IGNORING THE EFFECTS OF SETS OF ACTS It is natural to assume (The Second Mistake) If some act is right or wrong because of its effects, the only relevant effects are the effects of this particular act. This assumption is mistaken in at least two kinds of case.

IGNORING THE EFFECTS OF SETS OF ACTS It is natural to assume (The Second Mistake) If some act is right or wrong because of its effects, the only relevant effects are the effects of this particular act. This assumption is mistaken in at least two kinds of case. In some cases, effects are overdetermined.

Since neither X nor Y harms me, we are forced to the absurd conclusion that these two murderers do not act wrongly. Some would take this case to show that we should reject (C6). There is a better alternative. We should add (C7) Even if an act harms no one, this act may be wrong because it is one of a set of acts that together harm other people.

(C7) should be accepted even by Non-Consequentialists. On any plausible moral theory, it is a mistake in this kind of case to consider only the effects of single acts. On any plausible theory, even if each of us harms no one, we can be acting wrongly if we together harm other people.

I shall return to such cases in Section 30. There is a second kind of case in which we should consider the effects of sets of acts. These are co-ordination problems.

Key Concepts

• It is natural to assume (The Second Mistake) If some act is right or wrong because of its effects, the only relevant effects are the effects of this particular act.
• This assumption is mistaken in at least two kinds of case.
• Some would take this case to show that we should reject (C6). There is a better alternative.
• On any plausible moral theory, it is a mistake in this kind of case to consider only the effects of single acts.
• There is a second kind of case in which we should consider the effects of sets of acts.

Context

Opening of section 26, where Parfit introduces 'The Second Mistake' in moral mathematics and announces that it fails in at least two types of case: overdetermination and coordination problems.