8.1 Introduction to Personal Identity

What this lecture covers

This lecture opens the final unit of the course by separating two questions: what it is to be a person, and what makes a person at one time the same person as someone at a later time. It sets up the terminology needed for the rest of the unit, including the difference between things being merely similar and things being one and the same, and clears away a tempting but mistaken worry about how anything that changes over time could remain numerically identical to itself.

Key ideas

• Two questions about persons: what constitutes being a person (linked to questions about mind and body) is distinct from what constitutes personal identity over time.
• Similarity versus numerical identity: the word 'same' is ambiguous between qualitative similarity (two things having alike properties) and numerical identity (being one and the same thing), and Millican recommends using 'similar' and 'one and the same' to avoid confusion.
• Leibniz's law revisited: if A and B are numerically identical, any property of A must be a property of B; adding a baby who weighs less than a stone seems to contradict this if that baby is you today.
• Resolving the apparent contradiction: indexing properties to times, such as 'weighed less than a stone in 1958', removes the contradiction, since the same person can have different properties at different times.
• Hume's blind spot: unlike most of his otherwise sharp reasoning, Hume mistakenly thought strict identity required exact similarity, making change over time seem incompatible with remaining one and the same thing.
• Change as required for personhood: far from threatening identity, ongoing change, such as breathing, eating and thinking, is part of what it is to be a person at all.
• What might constitute identity: the lecture previews candidate answers, including an immaterial soul, physical or biological continuity, and psychological continuity, and distinguishes what constitutes identity from how we judge it in practice, such as using the body as evidence even if a soul is what really matters.

Before you watch

• This lecture starts a new unit, but familiarity with Leibniz's law from earlier in the course, where it was used to discuss mind and body, is helpful.

Check your understanding

1. What is the difference between the two questions 'what is it to be a person' and 'what is personal identity'?
2. Why does Millican recommend avoiding the bare word 'same', and what two terms does he suggest instead?
3. How does indexing properties to particular times resolve the apparent contradiction raised by Leibniz's law?
4. Why does the lecture argue that change over time is required for, rather than incompatible with, being the same person?