if we are nothing more than physical beings, how can we account for personal identity through time and change? In the last seven years—indeed, in the last five minutes—my body has undergone numerous changes. In what sense, then, am I the same person I was seven years ago or even five minutes ago? As J. P. Moreland and Scott Rae point out, a property-thing view of human persons means that when a person’s properties change, he changes. This is deeply problematic:
From that perspective, there is no essential person that survives the process of change. This would suggest that, for example, the person who committed a crime, the person brought to trial for the crime and the person serving a jail term for that same crime are all different persons. The notions of moral responsibility and criminal justice are both premised on a substance-dualist view of a person; otherwise, it would be conceptually difficult to hold anyone responsible for immoral or criminal actions.
Klusendorf then explains the superiority of substance dualism in accounting for human equality:
the substance view tells us that you are identical to your former fetal self even though you lacked a brain at that earlier stage of development. You are the same being now as you were then, though not because of something physical that will change over time, such as your brain function. From the moment you began to exist (conception), rather, you possessed a nonmaterial human nature that grounded your identity through all the stages of your development. There’s been no substantial change to your essential being even though your physical body has changed dramatically. Thus, if you are intrinsically valuable now, you were intrinsically valuable then as well.
Second, the substance view can account for human equality. Human equality is not grounded in some accidental property that may come and go in the course of our lifetimes (such as our immediately exercisable capacity for self-awareness), but in our common human nature. Put simply, the substance view says we’re valuable because of the kind of thing we are rather than some function we may or may not exercise.
Secularists...can deny this, but only at a terrible cost. For example, at any given moment, some of us can exercise greater brain function than others. In what sense, then, are we equal? Suppose my brain is severely damaged due to a stroke. Am I any less me? If I later regain all of my cognitive functions, am I back to my old self?
A simple way to remember this is to remember the L in the acronym SLED. L stands for Level of Development, and the level of development of a human being is irrelevant in providing them with moral value. The value of a human being is grounded simply in what they are not in any function they may gain and lose during their life. For more on SLED, see here.
Stand firm in Christ and stand firm for the preborn,