Who am I? What makes me, me? The theory of personal identity has been considered one of the most complicated questions that philosophy has taken a deeper look into. The theory of personal identity tries to "deal with the questions about how an individual at one time and place is the same individual at another time and place" [i] . When we look at personal identity, we are trying to figure out what sets us apart from other pieces of matter, species and individuals. In this essay I will discuss John Locke's criterion of sameness of consciousness for the theory of personal identity and why he does not think that other physical or psychological criteria fit, a challenge to Locke's criterion posed by Thomas Reid using the logic of transitivity analogy, and lastly how the suggested idea of overlapping chains of memory that Derek Parfit poses to reform Locke's view so that it may meet the challenge posed by Thomas Reid is a good solution to the challenge because it keeps Locke's theory intact and also adds the transitivity property that Reid challenges Locke's theory on.
When discussing personal identity, the criterion of identity will strongly depend on the object that you are talking about. There have been many viewpoints as to what the criterion of personal identity of the self or a person is. Some viewpoints suggest that the criterion for the self is organic; that our body is what identifies us over time. Locke states that this criterion is not applicable for personal identity of a person because of situations like body switch (Freaky Friday situation). If the consciousness of two people were to switch, everyone would think that you are the same person because you look like the same person. However, it is not actually you in your body and because there is a different consciousness in your body, and your body is no longer you; your body cannot define your personal identity. Other views describe the criterion as being substance that makes us the same over time. Substance includes both non-physical (soul) and physical substances. According to Locke, substance as a criterion is not applicable for personal identity of a person because of situations like death. When you die, you may be made of the same substance, but if you cease to think (thinking is connected to being a person) then you cease to exist and have no personal identity.
Locke looks towards a psychological criterion to define personal identity of a person. According to Locke, a person or the self is different than just matter and just a living thing. A person is "a thinking intelligent being that has reason and reflection and can consider itself as itself, the same thinking thing in different times and places; which it does only by that consciousness which is inseparable from thinking" [ii] . This describes what is known as sameness of consciousness. We are the same person to the point where we are conscious of our past and future memory or mental state in the same way that we are conscious of our current memory or mental state. This criterion can also be broken down into an analogy said by Locke: person x is the same as person y if person y is consciously connected to x in the sense that person y can remember the thoughts and actions of person x. According to Locke, the necessary criterion of personal identity is sameness of consciousness. This sameness of consciousness comes directly from memory and experience. Locke believes that the sameness of consciousness is the ideal criterion of personal identity for a person because since consciousness is always connected to thinking, and being able to think is what makes a person a person and allows that person to distinguish its own thoughts from another's, as far back as our consciousness can be extended to any past thought, so far back will our identity extend. As far back as the person can repeat or reflect on a past thought or action with its current consciousness, so far back is the personal identity of the person.