This is a Q & A blog post by our Visiting Scholar in Philosophy, William Lane Craig.
Dear Professor Craig, my name is Lucas, I'm sixteen years old and I'm from Brazil. Studying the field of philosophy of mind, I have come to a rather intelligent and yet perhaps problematic questioning of substance dualism. My question is: obviously I'm not strictly speaking the same person sixteen years ago, because when I was only one year old, prima facie, I was different from now. So I went through a kind of “evolution.” But evolution presupposes the loss and acquisition of different parts. But if the soul loses and receives parts, then it has parts. But if it has parts, then it is material. If it is material, then substance dualism is incorrect. Personally, I'm a substance dualist, but I don't know how to resonate with this argument. I hope you [can] help me.
Dr. Craig’s Response
Your question piqued my interest, Lucas, because I have just moved on to the locus Doctrine of Man in my study for my projected systematic philosophical theology. A key issue will be whether or not anthropological dualism is true. Anthropological body/soul dualism certainly seems to be an implication of the teaching of Scripture, particularly concerning the intermediate state between the death of the body and the resurrection at the return of Christ. So we need to consider whether or not there are insuperable objections to such a view.
Now your objection concerns the possibility of identity over time despite intrinsic change. Extrinsic change is no threat to identity over time, since something could remain intrinsically changeless even though its relations to things around it change. For example, I could become shorter than my son, not by my changing height, but by his changing height, while I remain the same. The question is, can there be non-relational change in something that retains its identity over time?
You seem to take it for granted that if you undergo intrinsic change, then you are “not strictly speaking the same person.” That seems to me patently wrong. You are clearly the same person who sent me this question, and I am the same person who began answering it moments ago, and any philosophical claim that denies this fact is crazy. I am strictly and numerically the same person, even though I have changed in various ways. Undoubtedly, I have changed; but equally undoubtedly, it is I who have changed and not two different persons. In saying that “I went through a kind of ‘evolution’,” you actually imply that you retained your identity throughout that change. If we take tenses seriously, we can remove any appearance of contradiction: I once was a child, but I am now an adult. Thus I do not possess incompatible properties, which would threaten my identity. Rather, I had different properties in the past than I have today.
I agree with you that the soul does not have parts and therefore is not material. It is a simple spiritual substance not composed of parts. But intrinsic change need not involve “the loss and acquisition of different parts.” There might be a change of qualities or properties rather than a change of parts. For example, when one’s soul undergoes a change from being stupid to being intelligent, it does not involve the acquisition of new parts! So you needn’t worry that intrinsic changes in the soul imply its materiality.
- William Lane Craig