When answering the above question we must analyse the nature of personal identity over time. The problem of personal identity is problematic, as we need to analyse and distinguish what exactly makes a person. We firstly need to distinguish between the body, the brain, personality, the mind and the soul. Some of these provide better identity criteria than others, as the existence of such entities as a soul are hard to prove. The two main groups we can identify a person under are that of a physical identity, and that of a psychological or mental identity.
We must also be aware of the distinction of numerical and qualitative identity. The two forms of identity are both problematic when relating them to personal identity. Numerical identity requires us to be numerically exact when comparing two people over time. If we assume that a person is fundamentally a human body then we would use numerical identity to determine if we are indeed the same person we were five years ago. Biologists would perhaps suggest that we are simply a complex series of matter brought about by evolutionary processes.
Our thoughts are nothing more than chemical reactions in the complex matter of our brains and there is no distinction between our thoughts and our bodies. Our brains are simply parts of our bodies, but we are under the illusion that the two entities are different. This theory is hard to reject, we may think our thoughts are independent but there are cases when it is clear our thoughts are governed by our bodies. When we use mind-altering drugs, our views of the world can change due to the different chemicals in our brains. We may start seeing things that aren’t there but we are convinced they are.
If it is possible for this to happen, then surely our minds are simply parts of our bodies, susceptible to change and alternation just like any other part of our body. However this mind/body problem can be troublesome, so I shall assume that we are indeed a physical biological system. However this can lead to problematic cases when we use this assumption as a matter of personal identity. If we are to assume that we are biological systems then there is no alternative but to use numerical identity. This, however, can be problematic.
If we are physical beings, then we are simply extended wholes with parts. When enough parts are lost and replaced, then we are no longer the same thing. This is troublesome in the case of humans as we reproduce cells and our bodies are constantly changing through time. The group of cells that constituted a baby are totally transformed, as it develops into adulthood. New skin grows, bones get larger and join together, the cells in our body die out and are replaced with new cells. There can be virtually nothing in our body that stays constant as we grow into adulthood.
Therefore if we assume that the body is this physical and biological matter, then it would be impossible to identify a person over time. The assumption that we are physical beings gives rise to further problematic cases, such as the problem of when do we become different beings to what we were 5 years ago. For example, if I were a collection of 100 parts, then if 2 or 3 parts changed I would still assume that I am the same person. Yet if 99 parts changed I would assume that I am a different person. However a problem would arise in trying to draw a line between when I am still myself and when I am a different person.
This may be a rather simplistic way to view the human body, as we are not simply a number of identical pieces, but it is useful in illustrate the problem that it is difficult to decide when a person’s identity transforms if we are simply physical objects. Since it is difficult to view changing identity over time if we under the assumption we are physical objects, we could assume that a human person is fundamentally a mind. We can assume that the body and mind are two separate entities, and the question remains are we the same mind as we were in the past.
In order to identify whether or not our minds change over time, we can use three types of criteria. The first two types of psychological identity are memory and intention. We can remember ourselves as a human being five years ago, and we could use our memories as a basis for our identity over time. However, using memory as a basis of identity is troublesome as our memories are never reliable. Our memories fade over time, so that we only remember small things of our past. It is virtually impossible to remember whole portions of ones past.
Also our memories may be distorted by what we want to believe rather than what actually happened. Even when we construct our memories we are still constructing them in the present, and these may be distorted by our current state of mind. Memories such as “I wore a red shirt that day” can be proved either wrong or right, but memories of emotions are more vague and unreliable. If we are to try to remember whether our mind was the same as yesterday it is virtually impossible. Cases of amnesia and par-amnesia also raise troublesome issues when trying to use memory and intention as a basis over time.
When we loose our memories through amnesia, does our identity change? If we are a sum of our past experiences then surely this is the case, as we wont be able to remember anything we have learned. Our characteristics may stay the same due to our instincts remaining intact, but our ways of thinking are surely going to change. If our ways of thinking is subject to total change, then surely our identity is also going to change. And even if the case is not as severe as amnesia, we still generally suffer from memory lapse so this too would mean that we don’t stay constant over time.
Therefore, it is not possible to use psychological criteria such as memory and intention in conjunction with identity over time. If this is the case then we could assume that even our minds aren’t a reliable source and could be drawn back to assuming that we are mere physical objects. However, we have already seen that this assumption is difficult. If we were to believe in the idea that the mind and body are separate entities, we would be adopting the idea of dualism. This theory, which was put forward by Rene Descartes in his Cartesian works, where he suggests that humans possess a soul outside of their physical body.
This is rather convenient as the soul can be the one constant needed to identify a person over time, but the existence of a soul is difficult to prove by science. But Descartes claims that the existence of the mind is the only thing that one cannot doubt. His claim is rather neatly placed into the Latin phrase “Cogito ergo sum”, which translated means “I think therefore I am”. However, although this phrase can go someway to convincing the existence of the mind at present, it can still not sufficiently answer the problem of a person’s identity over time.