In the first lecture on the mind, we just discussed the mind, period, no qualifications. In the second lecture there was a little more focus specifically on consciousness, and I noted that we have to be careful about what our arguments regarding the mind really prove. Descartes started out thinking he had proved the immortality of the soul, then backed off that claim, and many philosophers are content to argue that consciousness is non-physical, while admitting that the rest of the mind is physical.

So now we’re going to take the mind from a different angle, or take the question of what humans are from a different angle. I state the problem both ways, because some philosophers would classify the things I’ll be talking about as not “philosophy of mind” but “metaphysics”–perhaps because some elements could be applied to things without minds.

Let’s begin: you would all agree it would be wrong for one of you to kill me because you didn’t like your grade on the first midterm–well, I hope you agree with that. But it’s OK to kill a cabbage to eat it. Why? A common sense answer is that I’m a person, and as a person I have a right not to be murdered, but a cabbage isn’t a person, so a cabbage has no such rights.

Similarly: A person is convicted of murder, and is sentenced to prison for life. Why are we putting him in prison, as opposed to someone else. Here’s an answer to common-sensical as to be trivial: we think he’s the guy who committed the crime, and no one else is the guy who committed the crime. Or, say you’ve got a company with a corrupt CEO, and he sets himself to make millions of the company’s bankruptcy, ruining the employees who’ve heavily invested in the company’s stock. It seems like they deserve some of the money the CEO is trying to make off with. Why? Because they’re the ones who earned it, and not him.

Now: how do we account for this? How do we account for the fact that humans are people and cabbages aren’t? And how do we account for you being the person you were yesterday, and not the person the guy sitting next to you was yesterday?

Surprise surprise, this takes us back to dualism. Some dualists claim that only by postulating a soul can we answer this question. Why? Well, it seems like if personhood and personal identity is dependent on some specific bits of matter, or maybe the way matter is arranged into brain structures, we’ll get ambiguous cases. When we look a these ambiguous cases, this problem feels very real, though it may begin to cast doubt on dualism. Consider first the cabbage/human distinction. The theory of evolution says that if you go far enough back in time, you will find a population of organisms that gave rise to both cabbages and humans, and change into cabbages and humans came through a slow progression, with each step in the progression only slightly different than its parent. Dualist solution: and some precise point, souls were thrown into the mix, and that what makes personhood. I know a lot of religious people don’t like the theory of evolution, but the Catholic Church, for example, accepts evolution, and seriously believes that at some point in the history of evolution, God said “this population of organisms over here, I’m going to give them souls,” and BAM! we got people. And even if you don’t think there was actually a human-ape intermediary, it’s very plausible that there could be one–humans and apes look quite a bit different, but we have almost all the same molecular biology, almost all the same cell types, tissues, organs, systems, even brain organization: same brain regions for sensation and motion, at least similar brain regions for higher-level thinking, maybe humans have specialized brain circuitry for learning language that apes don’t, but the vast majority of stuff in humans and apes is the same, so an intermediary is very plausible. So maybe dualism is needed to give us a real distinction.

Or, consider development. The development of a baby is a complicated, drawn-out process, it’s hard to point to any point in scientific terms where you become a person. Anti-abortion activists like to say that it’s a scientific fact that life begins at conception, that’s not true, what’s a scientific fact is that sperm and egg cells are already alive, as are the blood and tissue samples that doctors sometimes take without thinking twice about. Conception itself isn’t even a single instant, a single Planck time for you physics majors in the class–conception too is a process, involving changes in the cell membranes and migration of genetic material. And it can’t be genetic code that makes a person, because of identical twins. Again, the Catholic church seriously believes that there is an exact moment in conception where God puts a soul in the fertilized cell, though I don’t know the details of official Catholic doctrine, as to how much the cell membrane has to change, or what God does about identical twins.

Finally, instead of the end of life, consider damage to life, specifically the brain. There have been lots of cases of brain damage where people lose specific capacities–they lose one area, they lose one capacity, they lose another, they lose another capacity. When do you stop being a person? Maybe when your soul leaves your body.

The dualist claim, made by people like Richard Swinburne, is that dualism lets us handle these problem cases in a nice and tidy way. But does it really work? To answer this question, let’s start with the Hollywood-and-Notre-Dame-undergraduate view of the soul. As I said two lectures ago, part of this view is that when your soul is disembodied, it will pretty much have your mental characteristics. That means if the brain is destroyed, your disembodied soul will pretty much have your mental characteristics. But why, then, when peoplpe’s brains are damaged can it change their mental characteristics so much? The Hollywood-and-Notre-Dame-undergraduate picture seems to fit poorly with our empirical facts.

There’s an analogy that is sometimes brought in here, the radio analogy. In the radio analogy, it’s argued that if you damage a radio, maybe the signal is less clear, and if you destroy it, the signal stops, but that doesn’t mean the signal is originating from the radio. The problem with this analogy is that with brain damage, something more specific is going on. Brain damage doesn’t just cause bad eyesight or loss of motor control. It can cause loss of memory, loss of specific capacities other than sense and movement, and even personality changes. Each change is pretty specific to the region of the brain damaged. It’s clear that the brain isn’t just receiving signals from the soul, the brain is doing most of the work.

Other dualists, you should realize, will tell you that they’re not Hollywood script writers. They can allow that the brain is important to memory and personality. But… and though I haven’t heard any sophisticated dualist like Richard Swinburne admit this explicitly… it seems like this reduces the soul to a sort of metaphysical barcode, used for keeping track of moral status but not anything else. This isn’t an especially attractive view.

What’s the alternative? Well, somehow we manage to talk about physical objects being of a certain kind, without their having souls. And somehow, we manage to talk about a machine being the same machine it was yesterday, before getting a part replaced, again without the machine having souls. And I think it’s obvious that we can accept ambiguous cases, that this metal wastebasket could be reshaped to the point where its status as a wastebasket is ambiguous, or that I could replace so many parts in my computer that its identity with the computer I used to own is ambiguous.

This analogy doesn’t prove that personhood is like that. But I think it shows there’s an alternative to dualism. Furthermore, given what we know about evolution and development and brain damage, it seems like the correct alternative.

This is the point in the lecture where I have to tell you that there’s a third option: some people want to reject dualism, and but also deny there’s any ambiguity in these questions of identity. There are several ways to approach this, and the whole problem is best explained with an example known as the Ship of Theseus. Imagine you’ve got a ship. At some point, the ship has to be moved over land, and for this purpose it is disassembled and reassembled. And over time, it has its planks replaced, so that eventually all the planks have been replaced. It seems like in both cases, it continues to be the same ship. But what if both happened at the same time? If planks were preemptively replaced, but then the old planks were reassembled into a ship?

Some philosophers insist that there’s a right answer to this question. Some side with the original-parts ship, and some side with the replacement-parts ship. And some think up more complicated views, according to which the “closest continuer” is the right ship–say the replacement-parts ship is the ship, but if the replacement-parts ship didn’t exist, then the original parts ship would be the ship.

These philosophers would then apply the same reasoning to people. In cases of brain transplants, or Star Trek style transporters, or duplicator machines, they try to formulate a theory that will always yield an exact answer as to questions of personal identity.

I find these views rather hard to swallow, and debates about them involve lots of subtle principles about logic and possibility and such. I don’t really want to inflict that on all of you, so let’s focus on one type of problem these views try to account for.

Consider the Star Trek transporters. In the show, it’s clear that these things are pretty powerful: they exactly analyze you, and beam your molecules across space, and then reassemble them according to their exact analysis of your molecular components. Maybe it’s assumed that they use the same molecular components, but they don’t really have to: in Star Trek, there are also replicators, which can make any food or whatever you want out of raw material, so probably the transporters could make a person out of raw material. Now, what if Scotty or whoever’s running the thing decides beaming the molecules themselves won’t work in some situation, so he’ll just disassemble you and reassemble a copy down on the planet. Is it really you? It seems like it can’t be, because then we’d also have to admit he could assemble two people who’d really be you, but they wouldn’t be identical to each other.

That is the sort of reasoning used in philosophical debates about personal identity. I have a rather hard time taking it seriously. The idea of being killed and replaced by a duplicate sounds horrifying, but so long as we remember it’s an exact duplicate, it’s hard to take seriously the idea it would make any difference in how we act. If someone you knew well was replaced by an exact duplicate, would you spurn your relationship with them because they weren’t really the same person.

Of course, this doesn’t do away with the pesky difficulty of having two duplicates around at once. But the reason that’s such a clear problem is that you can’t treat two people exactly as you treat one. There’s actually been at least on Star Trek episode where the duplicate problem arose, and the solution was just to put one duplicate on another ship to avoid the awkwardness–it was a practical problem. Now, Star Trek ignores the fact that once you’ve made one duplicate, people might figure out how to replicate the accident whenever it suits there purposes, and that’s a practical problem on a much grander scale. But it’s still a practical problem.

That’s my defense of a sort of inexact view of persons and identity. You should be aware, though, that this isn’t just a theoretical debate with no practical consequences. The nice, black and white metaphysical distinctions we thought we had were matched to nice, black and white moral distinctions. Give up one, and in some sense we must give up the other. We must accept something like moral gray areas on issues like abortion, animal welfare, and the treatment of people with severe brain damage. We also lose absolute principles about who deserves what, given their past actions.

There’s another side to the moral consequences of this view, though, that isn’t as depressing, though it is surprising. We have a tendency to worry an awful lot about our own welfare, to the exclusion of that of others, so much so that we’d really like to live forever. But given that personal identity is absolute, what does it matter if we live for ever? If I lived forever without some special guarantee of constancy in my personality, it’s likely that in a thousand years I would be just as different from my present self as a person born a thousand years in the future would be different from my present self. It’s a useful thought, when struggling with selfishness and fear of death.