Common-Sense Morality makes Parfit’s 'Second Mistake in moral mathematics'—it ignores the effects of… — Derek Parfit

Common-Sense Morality makes Parfit’s 'Second Mistake in moral mathematics'—it ignores the effects of sets of acts, i.e. what we together do—and thus, especially in cases where it is directly collectively self-defeating, it wrongly assumes that it is enough to consider only the effects of each person’s individual act.

By Derek Parfit, from Les raisons et les personnes

Key Arguments

Parfit explicitly identifies the error: 'This moral theory makes what I called the Second Mistake in moral mathematics. It ignores the effects of sets of acts—the effects of what we together do.'
He notes that Chapter 3 already established that ignoring the effects of what we together do is a mistake: 'Chapter 3 showed this to be a mistake.'
He explains how Common-Sense Morality embodies this mistake: 'Common-Sense Morality ignores these effects whenever it is directly collectively self-defeating. It tells each to do what will best achieve his M-given aims. This claim assumes that it is enough to consider the effects of what each person does.'
He shows what goes wrong under this assumption in self-defeating cases: 'In these cases, if each does what best achieves his M-given aims, we together cause the M-given aims of each to be worse achieved.'
To clarify the structural problem, he offers an analogy: 'This is like a case where, if each does what harms no one, we together harm many people.'
From this analogy he draws the normative conclusion that focusing only on individual effects is mistaken: 'In such cases it is a mistake to think that what matters morally are only the effects of what each person does.'

Source Quotes

This provides a sense in which our morality itself tells us to accept this revised version of itself.59 There is a further ground for thinking that we ought to revise Common-Sense Morality. This moral theory makes what I called the Second Mistake in moral mathematics. It ignores the effects of sets of acts—the effects of what we together do. Chapter 3 showed this to be a mistake.

Those who reject C would agree that, in some of my examples, we should not ignore the effects of what we together do. Common-Sense Morality ignores these effects whenever it is directly collectively self-defeating. It tells each to do what will best achieve his M-given aims. This claim assumes that it is enough to consider the effects of what each person does. In these cases, if each does what best achieves his M-given aims, we together cause the M-given aims of each to be worse achieved.

This claim assumes that it is enough to consider the effects of what each person does. In these cases, if each does what best achieves his M-given aims, we together cause the M-given aims of each to be worse achieved. This is like a case where, if each does what harms no one, we together harm many people.

In these cases, if each does what best achieves his M-given aims, we together cause the M-given aims of each to be worse achieved. This is like a case where, if each does what harms no one, we together harm many people. In such cases it is a mistake to think that what matters morally are only the effects of what each person does.

This is like a case where, if each does what harms no one, we together harm many people. In such cases it is a mistake to think that what matters morally are only the effects of what each person does. We must agree that this is a mistake even if we reject C and accept Common-Sense Morality.

Key Concepts

This moral theory makes what I called the Second Mistake in moral mathematics. It ignores the effects of sets of acts—the effects of what we together do.
Common-Sense Morality ignores these effects whenever it is directly collectively self-defeating. It tells each to do what will best achieve his M-given aims. This claim assumes that it is enough to consider the effects of what each person does.
In these cases, if each does what best achieves his M-given aims, we together cause the M-given aims of each to be worse achieved.
This is like a case where, if each does what harms no one, we together harm many people.
In such cases it is a mistake to think that what matters morally are only the effects of what each person does.

Context

Section 39 ('WHY WE OUGHT TO REVISE COMMON-SENSE MORALITY'), where Parfit is arguing that Common-Sense Morality should be revised to his theory R; here he appeals to its committing the 'Second Mistake in moral mathematics' by ignoring the effects of what people together do in directly collectively self-defeating cases.