Argument for substance dualism: Identity through change
Before I get into this whole thing about substance dualism, I think you might like to know a little background about me. For a long time, I didn't believe we had immaterial souls or spirits that left our bodies when we die. I didn't think a strong case for it could be made from the Bible. What changed my mind was not Biblical arguments, but philosophical arguments. Specifically, it was chapter 3 of Scaling the Secular City by J.P. Moreland that changed my mind. Most of my arguments are basically the same as his, although one is borrowed from chapter 3 of C.S. Lewis' book on Miracles.
The first argument goes like this:
1. If all we are is the sum of our physical parts, then we do not maintain identity through physical change.
2. We do maintain identity through physical change.
3. Therefore, we are more than the sum of our physical parts.
In other words, our identity consists of something immaterial, because it's the self, the ego, the identity of the person that endures through physical change.
If all we are is the sum of our physical parts, then we do not maintain identity through physical change.
To support the first premise, I'll use a thought experiment. Imagine that you have this old car that's beginning to break down. You keep having to replace parts on it. Each time you replace a part, you put the old part in storage. You keep replacing parts, one at a time. You even replace the frame at some point. Eventually, you have replaced every single part on the car such that none of the original parts remain. All the original parts are in storage. Now here's the question: Is this the same car with new parts, or is it a new car altogether?
If you say it's a new car altogether, good. That's what I wanted you to say. You can skip the next paragraph.
If you say it's the same car with new parts, then let me press the thought experiment further. Let's say you take all the original parts out of storage and reassemble them. Now you've got two cars--one with all original parts, and one with all new parts. Which car is the original car? Hopefully you can see by now that the one with all new parts is not the original car, because nothing of the original car remains in it.
The point of this thought experiment is to show that physical objects to not maintain identity through physical change. If you change all the parts, you've got a whole new object.
Now I'm not going to go into the question of when it becomes a new object. There's room for debate on that. Some people say it's a new object once you replace one part. Some say it's a new object once you replace more than half of the parts. Some say it's a new object once you replace the last of the original parts. It doesn't matter in our case. We know that sometime between replacing the first part and replacing the last part, it becomes a whole new object.
Now apply this principle to people. The cells in our bodies are constantly dying, dividing, and being replaced. It takes about seven years for every cell in your body to be replaced. That means every seven years, you've got a new body.
I should say at this point that brain cells live much longer, but even in the case of brain cells, the molecules that make them up are constantly being replaced, so even brain cells are in a constant state of flux.
If all we are is the sum of our physical parts, then we are a different person within every seven years. A sixty-seven year old women does not have the same body she had when she was two years old. If all she is is her body, then the sixty-seven year old woman is not the same person as the two year old person she is causally connected with.
We do maintain identity through physical change.
To support the second premise, we have to appeal to common sense. This situation is analogous to our belief in the uniformity of nature, that our memories correspond to a real past, that our perceptions correspond to a real world, and that our conscience corresponds to a moral law. None of these things can be demonstrated by proofs, but they have a strong appeal to common sense. It's the same with our identity. We know that we have existed for longer than seven years. I myself was once three years old.
To deny that we have an enduring self is counter-intuitive. It's so counter-intuitive, in fact, that those who deny an enduring self cannot live consistently with that denial. They still talk about things that happened over seven years ago as if they themselves were there. And they still hold grudges against people who wronged them over seven years ago as if it was really the present person who is responsible. And few of them would advocate an across-the-board 7-year statute of limitations on all crimes, so that nobody could be prosecuted for a crime committed more than seven years ago.
Therefore, we are more than the sum of our physical parts
This follows by modus tollens.
If A, then B.
Therefore, not A.
We are, in essence, immaterial selves. While we are intimately connected with our bodies, our identity rests with the immaterial self. This immaterial self is what we mean by "soul" or "spirit."
At this point, computer objections usually come up, but I'll save common objections for another post.