Key Arguments

• Parfit formulates Williams’s possible revision: 'The New Physical Criterion: A future person will be me if and only if this person is both living and has more than half my brain.'
• He concedes it meets Requirement (1): 'It is an intrinsic feature of this relation that it can take only a one-one form. It is logically impossible for two future people both to have more than half my brain.'
• However, identity would now hinge on whether the future person has 'half of my brain' or 'slightly more than half'; Parfit says this is 'a trivial fact', especially 'for those who believe that what matters is physical continuity', making the criterion violate Requirement (2).
• He offers a second, independent objection: someone could lose more than half his brain’s functioning and yet retain an unaffected mental life, with 'Less than half a brain ... enough to provide full psychological continuity'; it is 'hard to believe' and 'a second strong objection' that such a case would count, under the New Physical Criterion, as the original person’s ceasing to exist and being replaced by 'a new person who is merely exactly like him.'

Source Quotes

This criterion therefore also violates Requirement (2). Williams might suggest The New Physical Criterion: A future person will be me if and only if this person is both living and has more than half my brain.50 It is an intrinsic feature of this relation that it can take only a one-one form. It is logically impossible for two future people both to have more than half my brain.

This criterion therefore also violates Requirement (2). Williams might suggest The New Physical Criterion: A future person will be me if and only if this person is both living and has more than half my brain.50 It is an intrinsic feature of this relation that it can take only a one-one form. It is logically impossible for two future people both to have more than half my brain. This criterion therefore meets Requirement (1). It fails, however, to meet the other requirement.

I could be fully psychologically continuous with some future person both when this person has half of my brain and when this person has slightly more than half. And, for those who believe that what matters is physical continuity, the difference between these cases must be trivial. The second involves the continuity of just a few more cells. It is a trivial fact whether some future person has half my brain, or slightly more than half. The New Physical Criterion therefore violates Requirement (2). There is another objection to this criterion.

Less than half a brain would be enough to provide full psychological continuity. We would naturally believe that such a person survives his injury. But, on the New Physical Criterion, we must claim that such a person ceases to exist. The person in his body is someone else, a new person who is merely exactly like him. This is hard to believe. It is a second strong objection to this criterion. In all of its possible versions, the Physical Criterion faces strong objections.

Key Concepts

• Williams might suggest The New Physical Criterion: A future person will be me if and only if this person is both living and has more than half my brain.50
• It is an intrinsic feature of this relation that it can take only a one-one form. It is logically impossible for two future people both to have more than half my brain. This criterion therefore meets Requirement (1).
• And, for those who believe that what matters is physical continuity, the difference between these cases must be trivial. The second involves the continuity of just a few more cells. It is a trivial fact whether some future person has half my brain, or slightly more than half. The New Physical Criterion therefore violates Requirement (2).
• We would naturally believe that such a person survives his injury. But, on the New Physical Criterion, we must claim that such a person ceases to exist. The person in his body is someone else, a new person who is merely exactly like him. This is hard to believe. It is a second strong objection to this criterion.

Context

Later in Section 91, after rejecting the 'enough‑brain unless branching' criterion, Parfit considers a 'more‑than‑half‑brain' New Physical Criterion, acknowledges its formal advantage, and then argues that it makes identity depend on trivial or highly counterintuitive facts.