I believe in myself as a being. I believe that when I was six years old, I was me, and now, many years later, I am still me. Now, obviously I physically exist, because otherwise something very funny is happening to my keyboard. But that’s not what I’m talking about. I mean that there is a quality of ghjm-ness which is shared between six-year-old me and however-old-I-am-now me. Let’s call this property X.

I do not insist that X should be a supernatural or spiritual property. I don’t know anything about X other than the above. So what is the nature of X?

X cannot arise from my particular composition atoms and molecules, because all or very nearly all of my atoms and molecules have changed since I was six. X cannot arise from my memories, attitudes, moods or ideas, because I remember very few of the things that occupied me when I was six, and my moods, attitudes and ideas have changed quite drastically since then.

X cannot arise from the chemistry or ion channel interactions in my brain, because those are in a constant, moment-by-moment state of change. Nor can X arise from an arrangement of neural pathways, even if they are relatively long-lived, because X would still hold – I would still be me – even if I suffered some horrible brain injury that disrupted these pathways, and even if such an injury radically changed my moods, attitudes or capabilities.

But perhaps X is an emergent property of some non-obvious physical attribute. Maybe I am “me” due to some informational content of the “software” of the brain that will in the future be explained by science in strictly materialistic terms. So, in the great tradition of scientists who don’t have any data yet, let’s do a thought experiment. Suppose we had a matter duplicator. Through some future physics, this duplicator is able to reproduce, precisely, a designated object of matter, all the way down to its quantum states. With this matter duplicator, I can create a living copy of myself.

As the operator of the duplicator, I can be quite certain that I am still me. The new copy, walking out of the duplicator, is not. Because he shares my memories, attitudes and beliefs up to the point of duplication, he, like me, will insist on his own enduring existence – but he will insist on Y, which he will agree is different from X. Even though no memories, neuronal pathways, brain chemistry or “soft” states have been destroyed, or, at the moment of duplication, changed, he is simply not me. If at the conclusion of the experiment, I decide to clean up my lab and set about destroying the “temporary” copy, I am sure he would strenuously object. He would see Y as being just as valuable as I see X. What is he protecting? The value he ascribes to Y cannot arise from his two minutes of unique existence in the lab. Both of us would agree to lose those memories for a million dollars, but we would not agree to be killed for a million dollars. So there is some essential value of Y other than its slight difference from X.

From this, it appears to me that insisting on my own existence is incompatible with strict materialism, even if you deny (as I do) the existence of God or (in the Christian sense) souls.