1. Personal Identity What makes each of us the same person over time?

2. John Locke (1632-1704)  An early British Empiricist, wrote An Essay Concerning Human Understanding (1690)  An important enlightenment figure.  Developed influential views on the ‘social contract’ and ideas about property.

3. The puzzle of identity  We all change over time.  What changes certainly doesn’t stay qualitatively identical, that is, it doesn’t have the same properties at all times.  But we also say that we remain one and the same person through our changes.  This notion of personal identity (a form of numerical identity) is important to much of what we do.  But it’s hard to say just what makes me the same person from one day to the next…

4. Locke’s criterion  Lots of things continue over time, despite changing in various ways.  Our bodies are an obvious example.  But what binds us together as one being over time, for Locke, is not sameness of body.  The same body could fail to ‘contain’ the same person. (cf. Sheckley, “Mindswap”)

5. What about souls, then?  Souls, or minds as Descartes might prefer, also seem to persist as one and the same thing over time.  So could possessing or ‘being animated by’ the same soul over time be what makes us the same person?  Locke rejects this response as well: what counts is not the substance in which our consciousness inheres, but the continuity of consciousness, whether in one substance or many: “Which… concerns not personal identity at all.” (366)

6. Locke’s view “For as far as any intelligent being can repeat the idea of any past actions with the same consciousness it has of any present action; so far it is the same personal self.” (366).  For Locke, this link of consciousness and memory is the basis for responsibility, too: “I being…as justly accountable for any action that was done a thousand years since, … as I am for what I did the last moment.”

7. Corollaries  Loss of memory (complete permanent) constitutes a different person.  A madman is not accountable for what he did when sane.  A sane person is not accountable for what she did when insane.  We have to be careful about our meanings here, since we tend to muddle together sameness of soul (same spirit), sameness of living body (same animal or man), and sameness of consciousness (same person).

8. Thomas Reid (1710-1796)  18 th Century philosopher, wrote against Hume’s skepticism.  Here, he pursues some of the odder implications of Locke’s link between personal identity and memory.

9. The General  Here we have overlapping sets of memories, connecting the child to the young man and the young man to the General.  This leads to an important logical point.  Identity has certain logical properties: Reflexivity: A=A. Symmetry: If A=B then B=A. Transitivity: If A=B and B=C, then A=C.

10. Memory vs. Consciousness  Reid imposes a distinction Locke isn’t concerned about:  When we know what we’re thinking or sensing or doing right now, that’s consciousness.  When we know what we thought or sensed or did at some point in the past, that’s memory.

11. Evidence and Fact  Another distinction Reid insists on:  Memory may be the evidence that supports our conviction that we are the same person now as the person who did something in the past.  But we shouldn’t confuse this with the fact this evidence is evidence for, i.e. with what makes us that same person: “Consciousness is the testimony of one faculty; memory is the testimony of another faculty; and to that that the testimony is the cause of the thing testified, this is surely absurd…” (369)

12. Consciousness  Third, Reid worries about the fact that consciousness, the ground (according to Locke) of our identity, is itself always in flux, always changing.  If consciousness itself has no continuing existence, surely it’s impossible to rely on it as constituting the identity of the person whose consciousness it is.

13. The puzzle of memory  Finally, Reid worries about what makes a certain ‘apparent’ memory a real memory.  If having a memory of doing x presupposes that you are really the person who did x, then ‘being the same person’ is implicitly built into the idea of memory, and using memory as the criterion for being the same person is circular.  Worse, dispensing with this requirement drops the idea of personal identity altogether, in favour of an idea of similarity (of a certain kind) between what must be different persons, due to the ever-changing nature of consciousness.

14. Daniel Dennett (1942- )  An influential philosopher of mind, he has also written on evolution and religion: Darwin’s Dangerous Idea, and Breaking the Spell.  Instigator of the Philosophers’ Lexicon.

15. Yorick, Hamlet and Points of View  Dennett undergoes a brain-extraction.  Then he wonders, ‘where am I?’.  The answer is surprisingly hard to sort out…