8.4 Persons, Humans and Brains

What this lecture covers

This final lecture in the personal identity sequence asks what happens if you try to identify a person with a developed, functioning brain rather than with the whole human organism or with continuity of consciousness alone. It follows directly from the previous lectures on Locke and on the objections to his memory-based criterion.

You will see why identifying persons with brains creates its own puzzles once split-brain cases are considered, how Derek Parfit responds by treating personal identity as a matter of degree, and how the lecture ends by connecting this puzzle to a more general idea about concepts breaking down in radically novel situations.

Key ideas

• Person versus organism, revisited: if being a person requires a significant mental life, then a fetus, and perhaps a body at the end of life, would be the same organism as you without being a person.
• Brain-based personhood: one response is to identify the person with the developed functioning brain, which fits cases like a Bernard Williams-style brain transplant.
• Split-brain cases: surgically separating the two hemispheres can already produce conflicting behavior between them, and hypothetically each half could be transplanted into a separate body.
• Two successors problem: if a split brain produces two people who each have brain and memory continuity with the original, strict either/or personal identity seems to break down.
• Parfit's degree view: Derek Parfit suggests treating personal identity as a matter of degree rather than all-or-nothing, tied to how much we care about and plan for a future self.
• Open texture: a concept coined for cases (attributed in the lecture to Waismann) where an everyday concept becomes genuinely unclear in a radically new situation, illustrated with how "marriage" or "having a conversation" strain under novel circumstances.

Walkthrough

Persons as an accidental property (0:11)

The lecture reopens the distinction between a person and a human organism. If personhood requires a significant mental life, then earlier and later stages of the same organism (a fetus, or a body after mental life has ended) may not count as persons, which makes being a person look like an accidental rather than essential property of you.

Identifying the person with the brain (2:13)

To avoid that discomfort, the lecture considers identifying the person with the developed, functioning brain rather than the whole organism. This fits well with thought experiments where a brain is "transplanted," since intuitively the recipient's body goes with the brain rather than the other way around.

Split-brain cases (3:14)

The lecture complicates the brain-based view with split-brain surgery, where cutting the connection between hemispheres can produce uncoordinated behavior. Taken further, each hemisphere could hypothetically be transplanted into a separate body, producing two people who both have real continuity with the original person.

Parfit's matter-of-degree view (5:16)

Faced with two equally good claimants to being the original person, the lecture presents Parfit's suggestion that personal identity should be treated as a matter of degree, tied to the forensic idea of how much you care about and plan for a future self, rather than as strict all-or-nothing identity.

Open texture and closing remarks (7:17)

The lecture ends with the idea of concepts that become genuinely unclear in radically novel situations, illustrated with marriage and the idea of "having a conversation." The suggestion is that personal identity puzzle cases may be exposing situations our ordinary concept was never built to answer.

Before you watch

• Watch the two earlier lectures in this sequence, on Locke's theory of personal identity and on objections such as Reid's problem, since this lecture builds directly on both.
• Be familiar with the brain-swap thought experiment introduced earlier in the sequence, since split-brain cases extend it.

Check your understanding

1. Why might identifying personhood with having a significant mental life make being a person an "accidental" rather than essential property?
2. What problem do split-brain cases create for the view that a person just is a functioning brain?
3. How does Parfit's suggestion that identity is a matter of degree respond to the two-successors problem?
4. What does the marriage example illustrate about concepts facing radically novel situations?