One muddle that materialists find themselves in has to do with the philosophical problem of personal identity. This is the problem raised by trying to assess what it is about us that makes us who we are. What is it about us that makes us uniquely us?
Consider the following thought experiment proposed by the late philosopher Derek Parfit in his book, Reasons and Persons:
A man walks into a teleporter, a device much like the contraptions on the old Star Trek television series which dematerialize a person in one location and rematerialize him in another. The man pauses for a moment while the device scans the physical state of every single molecule in his body.
In a few seconds, his entire physical being — his brain, heart, blood vessels, cells, genes, and even the contents of his stomach — will be annihilated and instantaneously recreated from new materials in another teleporter on Mars.
From his perspective, he'll arrive at his destination with no lapse in consciousness, no pain, not even a single hair out of place.
In what sense, then, is the person who materializes on Mars (call him person B) a different person from the person who was on earth (person A)? If person B is a different person from person A because B is made from different atoms, then are not we different persons than the person we were when we were conceived, or when we were born?
If we are different persons because the passage of time has caused us to incorporate new matter into our bodies such that no atoms remain that were in our bodies twenty years ago, suppose you take out a 30 year mortgage to buy a home. After twenty years go by why should you be responsible for paying the balance? In what sense are you the same person who took out the loan?
Suppose person A is not annihilated so that A continues to exist when B materializes on Mars. Are there now two versions of the same person? Imagine that A commits a crime. Is it fair to hold both A and B responsible for the offense?
These puzzles arise as a consequence of the initial assumption made by materialists that all there is to us is the matter that makes up our bodies along with whatever epiphenomena arise from that matter. But if, on the other hand, we are fundamentally an immaterial mind or soul that possesses a body and which somehow constitutes our essential self, perhaps this self remains unaffected by the dematerialization of the body since the scanning machine could only duplicate material substance but cannot duplicate immaterial mind.
If so, person B would lack a mind or soul and not even be a person but rather a sort of zombie replica of A.
Perhaps along these lines lies a solution or at least a partial solution to the teleporter problem and the problem of personal identity. In any case, materialism certainly can't solve the problem. Indeed, many of the vexing problems of philosophy, e.g. the free will problem, problems of ethics and of course the mind/body problem, arise because of the assumption that we are nothing other than material beings.