A common question that comes up when thinking about personal identity is: what makes the person I am now the same as the person who was me some time in the past? Clearly we aren’t identical, many things change over time, but we do want to claim that there is some “personhood” that is shared by us.
A common candidate for the property of “personhood” is psychological continuity. The me now and the me yesterday share many of the same desires, attitudes, memories (including new memories of being me yesterday), ect. Dualists would reject this view of “personhood”, of course, but fortunately we are not dualists, for reasons I have outlined elsewhere. The only other alternative would be to subscribe to some kind of physical continuity as defining who we are, but since our cells, and the proteins that constitute them, are constantly in a process of being replaced, without any noticeable effect on our “personhood”, I think we can safely rule it out as an alternative.
Two immediate consequences spring from defining personal identity in this way. One is that two people-instants (a person at a particular point in time) are not simply the same person or not. Psychological continuity comes in degrees, and thus our judgments as to whether two people-instants are the same person must as well. The me now and the me yesterday are the same person to a high degree, while the me now and the me several years ago are the same person to some lesser degree. The second consequence is that it is possible to kill someone by preventing them from sharing a psychological continuity with future people, even if their body lives on. For example, if we had a machine that replaced a person’s desires, attitudes, memories, ect with those of another person to use the machine on someone would be to kill them.
Now let’s consider a slight modification the person-replacing machine. Instead of replacing the person’s psychological attributes all at once it transforms their existing ones into those of another person over some short period of time. Clearly this process is equally destructive to the original person. But, as the title of this post foreshadowed, this is just what happens in ordinary life, since we change over time. For some people there is likely a point in their lives where the only thing they have in common with the “them” at some distant earlier point in their lives are perhaps some vague memories of that time. And, even if these vague memories provide some psychological continuity, it is no more than I have with someone else who, let us say, shares some of my attitudes and goals. And if the psychological continuity I share with the stranger isn’t enough to make us the same person then clearly the vague memories we might share with our earlier selves aren’t enough either.
Have we then killed our earlier selves? (Or perhaps our earlier selves committed suicide, since they didn’t object to the changes that turned them into us.) And even if we have it certainly wouldn’t make sense to fight this process, since that would force us to lock ourselves into the same attitude, opinions, and desires that we have now, definitely not a healthy idea. Now some might object to this conclusion, claiming that because there were intermediate stages that shared the necessary psychological continuity with our earlier selves then somehow our “personhood” has been transmitted intact through them. If this were the case then we shouldn’t resist having the person-replacing machine, modified to transform us over the space of an hour, used on us, since in that case too there are intermediate stages to preserve our “personhood”. Of course one might still resist if they disliked the person it changed you into, so let me stipulate that the person the machine changes people into is something that everyone would desire, and in fact strive for (intelligent, wise, sensitive, moral, ect). Even so, I think most people would resist having the machine used on them, because they feel that in some way it would eliminate the person who they are now, and if this is the correct interpretation then my original reading of psychological continuity holds, and thus the people some of us were in the past really have been replaced by new people. I don’t know exactly how we should read the moral implications of this, so I will leave them as an exercise for the reader.