Personal Identity

Identity over Time

Identity over Time

The Long-Lost Brother

The Persistence Question If a person x exists at one time and something y exists at another time, under what possible circumstances is it the case that x is y?

The Persistence Question If a person x exists at one time and something y exists at another time, under what possible circumstances is it the case that x is y?

The Persistence Question

The Persistence Question

The Persistence Question

The Persistence Question

The Persistence Question

The Persistence Question

The Persistence Question

Psychological Continuity

What Makes X and Y the Same?

What Makes X and Y the Same? X remembers the experiences of Y. Y X

What Makes Y and Z the Same? X

What If X Doesn’t Remember Z’s Experiences? Y X

A Logical Problem X = Y Y = Z But: X ≠ Z This violates the transitivity of identity.

What Makes X and Y the Same? Y’s mental states are causally responsible for X’s mental states Y X

What Makes Y and Z the Same? X

Causation Is Transitive Z Y X

So Can We Be Uploaded?

So Can Our Brains Be Transplanted?

So Can Our Brains Be Transplanted?

Fission

Derek Parfit (1942-2017) British philosopher Born in Chengdu 成都 (RoC) Studied history, no formal training in philosophy Wrote Reasons and Persons, one of the greatest books of Western moral philosophy in 20th C. Gold star decisions!

Brain Hemispheres Brain has two halves (hemispheres). You can survive with only one! Sometimes, for severe tumors, doctors remove one entire hemisphere. Although the different halves do some different things, the remaining half can take over the lost functions.

Important Points On the psychological- continuity view, you can survive brain transplantation. Given the remarkable nature of the brain, on the psychological-continuity view, you can survive half-brain destruction.

Personal Identity You can survive brain transplantation: If we put A’s brain in B’s body, then the result will be A.

Personal Identity Additionally, you can survive half-brain destruction. So if we destroyed half of A’s brain and put it in B, the result would still be A.

Personal Identity But what happens if we divide A’s brain in half, and put one half in B’s body and the other half in C’s body?

Three Possibilities A is neither of the resulting people. A is one of B or C, but not the other one. A is both B and C.

Possibility 1: A is Neither This solution is very bizarre. It says that if you transplant half my brain into another body, I still survive, but only if you destroy the other half. If you instead transplant the other half into another body, nobody is me, I don’t exist.

Possibility #2: A Is One, But Not the Other This solution is completely undermotivated. If A = B, then why is that so? Half of A’s brain is in B, but the same is true of C. A is psychologically continuous with B, but the same is true of C. Nothing motivates saying A = B rather than A = C and vice versa.

Possibility #3: A Is Both B and C. This solution is logically problematic. B C

Possibility #3: A Is Both B and C. If B = A and A = C, then by the transitivity of identity: B = C. B C

Possibility #3: A Is Both B and C. But arguably, B and C are different people. They have different experiences, different jobs, different friends (they share some memories). B C

Paradox? A is neither of the resulting people. Very Bizarre A is one of B or C, but not the other one. Completely Undermotivated A is both B and C. Logically Problematic

Possibility #4? There never was a single person A. The fact that A would eventually split into two means that A really was two persons all along.

Personal Identity If we charted the two people’s lives, it would look something like this:

Other Fusion Cases: Teletransporter Brain division and transplantation isn’t the only way to imagine a fission case.

Personal Identity Parfit suggests that we can talk of survival without talking about identity. If we are not worried about identity, we can simply say that if my mind divides, I survive as two people. But he doesn’t think there is an answer to the question of whether I am identical with those people.

Personal Identity Parfit thinks that we can talk about survival without taking on some of the problematic features of identity – for instance, the idea that identity is one-to-one, and all-or-nothing. So I can survive more or less.

Fusion

Brain Hemispheres The two halves of the brain share information through a dense network of nerves called the corpus callosum (“tough body”). Sometimes doctors sever the corpus callosum to treat severe seizures. This means the two halves can no longer communicate!

Visual Information Interestingly, the right side of the brain processes visual information from the left eye, and the left side processes information from the right eye. So when the corpus callosum is severed, the left brain knows what the right eye sees, but not what the left eye does.

Personal Identity If we set up a barrier between the patient’s two visual fields, we can display an image only to the left hemisphere, or only to the right hemisphere. If you display an image to the left hemisphere (on the right), and ask the person what they saw, they can describe it normally.

Personal Identity However, if you show an image to the right hemisphere and ask the subject what they saw, they say they saw nothing. If you ask them to point to a matching image instead, however, they succeed.

Personal Identity Why does this happen? Normally, speech centers are present only in the left hemisphere of the brain. Since the right half cannot communicate with the left, the left has no access to the right’s experience. But the experience occurred – the pointing shows that.

Personal Identity Another interesting experiment: the right hemisphere was shown two words one after another: ‘fire’ and ‘arm’. When asked to draw a picture for the words, the subject drew a rifle.

Personal Identity But when the two words were presented to different hemispheres, the subject drew a fire and then an arm!

Personal Identity What this shows is that the two hemispheres could not integrate their separate experiences – they could not combine the information.

Fusion Case?

Fusion Case http://www.youtube.com/watch?v=KBE3xaY6W4E

Fusion If it is possible for conjoined twins to share experiences, maybe it is possible for a single body to contain two persons, sharing all experiences until separated?