In an excellent lecture found here, Dr. J. P. Moreland, philosopher of mind and Christian apologist, makes three arguments in defense of dualism (the theory that humans are two substances--soul and body). Each of these arguments merits careful consideration.
1. The argument from divisibility: Dr. Moreland says,
Physical objects are divisible. That means they can come in percentages. I am not divisible. So, I am not a physical object. Persons do not come in percentages. Illustration: If you remove 55% of a person’s brain in a surgery, you are not left with only 45% of a person.
2. The Argument from Disembody-ability: Dr. Moreland Says,
There is something true of me that isn’t true of my brain and body. Namely, I am possibly disembody-able. But, my brain and body are not. Illustration: If I had little grains of salt and little pieces of marble that looked like little grains of salt, the salt would be possibly dissolvable but the marble would not be possibly dissolvable. This proves that the salt and the marble are not the same. We are possibly disembody-able. The brain and body are not. So, there is something true of me which is not true of my brain and body.
Dr. Moreland also states this argument in logical form in Philosophical Foundations for a Christian Worldview:
- The law of identity is true: If x is identical to y, then whatever is true of x is true of y and vice versa.
- I can strongly conceive of myself as existing disembodied.
- If I can strongly conceive of some state of affairs S that S possibly obtains, then I have good grounds for believing that S is possible.
- Therefore, I have good grounds for believing of myself that it is possible for me to exist and be disembodied.
- If some entity x is such that it is possible for x to exist without y, then (i) x is not identical to y, and (ii) y is not essential to x.
- My body (or brain) is not such that it is possible to exist disembodied, i.e., my body (or brain) is essentially physical.
- Therefore, I have good grounds for believing of myself that I am not identical to my body (or brain) and that my physical body is not essential to me.
3. The Argument from Free-Will: Dr. Moreland says,
I believe that I can act freely. That is, I can do something like raise my hand and I could have done otherwise. I could have refrained from raising my hand. Nothing made it such that I had to raise my hand. It was my choice. Moral responsibility requires human freedom. If am my brain, then my behavior is determined strictly by my brain chemistry, genes, and environmental inputs. If I am my brain, I am completely determined in behavior and I cannot act freely. But, I know that I act freely, at least, on some occasions. It follows, then, that I am not my brain. I am a soul.
If it is true, as it seems to be, that you and I are indivisible, disembody-able, and free, doesn't it make sense to think that we are souls? And, if we are souls, doesn't that change everything?!
 Moreland, J. P.; Craig, William Lane. Philosophical Foundations for a Christian Worldview (Kindle Location 7101). InterVarsity Press. Kindle Edition.