Although Q is plausible, it only applies to Same Number Choices and so does not by itself solve the … — Derek Parfit

Although Q is plausible, it only applies to Same Number Choices and so does not by itself solve the Non‑Identity Problem; because different higher‑level principles can underwrite Q yet diverge on Different Number Choices, Parfit concludes that we need a further, as‑yet‑unknown Theory X that both resolves the Non‑Identity Problem when different numbers of people exist and explains or justifies Q.

By Derek Parfit, from Les raisons et les personnes

Key Arguments

After endorsing Q, Parfit immediately limits its scope: 'Though Q is plausible, it does not solve the Non-Identity Problem. Q covers only the cases where, in the different outcomes, the same number of people would ever live.'
He notes the need for a more general claim: 'We need a claim that covers cases where, in the different outcomes, different numbers would ever live. The Non-Identity Problem can arise in these cases.'
He explains that Q’s restriction allows multiple incompatible justifications: 'Because Q is restricted, it could be justified in several different ways. There are several principles that imply Q, but conflict when applied to Different Number Choices.'
He draws the methodological conclusion: 'We shall need to decide which of these principles, or which set of principles, we ought to accept. Call what we ought to accept Theory X.'
He specifies the tasks for Theory X: 'X will solve the Non-Identity Problem in Different Number Choices. And X will tell us how Q should be justified, or more fully explained.'

Source Quotes

If she loves me, her actual child, this is enough to block the claim that she is irrational if she does not have such regret.10Even when it implies a claim like (3), I conclude that we can accept Q. Though Q is plausible, it does not solve the Non-Identity Problem. Q covers only the cases where, in the different outcomes, the same number of people would ever live.

Though Q is plausible, it does not solve the Non-Identity Problem. Q covers only the cases where, in the different outcomes, the same number of people would ever live. We need a claim that covers cases where, in the different outcomes, different numbers would ever live.

Q covers only the cases where, in the different outcomes, the same number of people would ever live. We need a claim that covers cases where, in the different outcomes, different numbers would ever live. The Non-Identity Problem can arise in these cases. Because Q is restricted, it could be justified in several different ways.

The Non-Identity Problem can arise in these cases. Because Q is restricted, it could be justified in several different ways. There are several principles that imply Q, but conflict when applied to Different Number Choices.

Because Q is restricted, it could be justified in several different ways. There are several principles that imply Q, but conflict when applied to Different Number Choices. We shall need to decide which of these principles, or which set of principles, we ought to accept.

We shall need to decide which of these principles, or which set of principles, we ought to accept. Call what we ought to accept Theory X. X will solve the Non-Identity Problem in Different Number Choices.

Call what we ought to accept Theory X. X will solve the Non-Identity Problem in Different Number Choices. And X will tell us how Q should be justified, or more fully explained.

Key Concepts

Though Q is plausible, it does not solve the Non-Identity Problem.
Q covers only the cases where, in the different outcomes, the same number of people would ever live.
We need a claim that covers cases where, in the different outcomes, different numbers would ever live. The Non-Identity Problem can arise in these cases.
Because Q is restricted, it could be justified in several different ways.
There are several principles that imply Q, but conflict when applied to Different Number Choices.
Call what we ought to accept Theory X.
X will solve the Non-Identity Problem in Different Number Choices. And X will tell us how Q should be justified, or more fully explained.

Context

Closing lines of Section 122, where Parfit highlights the limitations of Q and sets up the search for a more comprehensive ethical theory—Theory X—that can handle Different Number Choices and ground Q.