A few words from philosopher Shelly Kagan's tenth lecture in the online Yale course on Death. Complete course material found here.
The commentary here is on the soul theory of personal identity: can we use the soul to explain personal identity?
Come back to the soul view. It's me as long as it's the same soul. It's not me if it's a different soul. Now consider the following possibility. Suppose that over the weekend, at 3:00 a.m., Saturday night, Sunday morning, while I'm asleep, God replaces my soul with a different soul, hooks it up to the body, gives that soul, that replacement soul, all of my memories, all of my beliefs, all of my desires, all of my intentions. Somebody wakes up Sunday morning and says, "Hey, it's a great day. Wonderful to be alive. I'm Shelly Kagan. Got to get to work." Whatever it is. Says "I'm Shelly Kagan"; but he's not. According to the soul view, he's not. Because according to the soul theory of personal identity, to be me that person's got to have my soul. And in this story, he doesn't have my soul. My soul got destroyed, let's suppose, 3:00 a.m. Sunday morning. A new soul got created. It's not me. There's a person there, all right. It's a person that doesn't have a very long history. Maybe he'll go on to have a long history. But it's a different extended through space and time person than the one you're thinking about right now. Because, according to the soul view, to be me it's got to have the same soul and we just stipulated, not the same soul.
Think about what that means. If God were to replace my soul Saturday night, I die. And the thing that wakes up Sunday isn't me. Of course, he'd think he was me. He'd think to himself, "I'm the very same person who was lecturing about philosophy last week." But he'd be wrong. It isn't the same person, because it's not the same soul. He'd be wrong and — notice this — there'd be no way at all he could tell. He could check his beliefs. He can check his desires. He can check his memories. But that's not the key to personal identity, according to the soul view. The key to personal identity, according to the soul view, is having the very same soul. You can't check that. You can't see the soul to see if it's the same one. So if this were to happen to him, he wouldn't be Shelly Kagan, the person who'd been lecturing last week. But there'd be no way at all he could know that.
And now the question you would need to ask yourself is, how do you know this didn't happen to you last night? You woke up this morning thinking, I'm the very same person — Joe, Linda, Sally, whatever it is — the very same person who was in class yesterday. How do you know? How could you possibly know? If God replaced your soul with a new one, destroyed the old one, gave the new one all the old memories, beliefs, desires, goals, and so forth, that person who was in class last week, yesterday, died. The person who's here now hasn't been around 10 years, 20 years, what have you. You were born a few hours ago. And there'd be no way at all that you could possibly tell.
How do you know, not only that it didn't happen to you last night, how do you know something like this doesn't happen every single night, every hour on the hour, every minute, every second? God whips out the old soul, destroys it, puts in a new one with — Maybe souls only last for a minute and a half. If that was happening, then people don't last very long. Bodies may last 20 years, 50 years, 80 years, 100 years, but people would only last an hour or, if it's every minute substitution, a minute. And you'd never possibly be able to tell.
Now these worries were raised by John Locke, the great British philosopher, and he thought, this is too big a pill to swallow. This is too big a bullet to bite. We can't take seriously the suggestion that there's no way at all to tell whether it was still me from the one day to the next, from one hour to the next, from one minute to the next, just not plausible. It's not that there's anything incoherent about this view. It doesn't say anything logically contradictory about this view. You just have to ask yourself, "Could this really be what personal identity is all about? That there'd be no way at all to tell whether I've survived from one minute to the next, from one hour to the next?" Locke thought no, you couldn't possibly take this view seriously if you thought about what it meant.