It is very early in the morning. Standing in front of a mirror, I notice the appearance of a person. A few quick glances and I decide that the person’s looks are almost identical to that of a person I saw last night in front of the same mirror, and the morning and night before that. Believing this person will remain to be the same — especially given his track record of repeatedly convincing me that I am him — I become flattered to reveal some of my memories to him, with my thoughts dispersing over the various tasks I hope to accomplish during the day while sometimes being distracted by worries about what I could have done and fancies about what could happen.

Nonetheless, I remain firm in my belief that this person is loyal to the core of who I am and, hence, trustworthy enough for me to divulge my past experiences and future aspirations to him, and act in his best interest, despite the urge to be suspicious and confidential.

However, this is not the usual way of going about business. We, humans, rarely doubt who we are. For whatever reason, we usually seem to be pretty sure about our identity as we live our daily lives. Nonetheless, it is worth asking why. Why do we take who we are for granted? Yet, a more interesting question would be how. How do we know who we are? How did I know the person standing in front of the mirror was me? In most cases, despite the centuries-old warning, we judge identities based on physical appearances.

Take the example of an old palm tree. What does it take to be this particular tree? Of course, it would need its unique leaves and long trunk. But is that enough? A thorough account could include the particles that make up the leaves and the trunk, as well as how they come together to produce a functional system that nourishes the palm tree and maintains its existence.

Yet, the tree has been passing through several stages of life, from a seedling to a young palm and through maturity to an old palm, all the while losing and gaining its constituent particles. What kind of account would identify one particular palm tree? Of course, we never talk about how a palm tree expresses itself nor about its past lives, as we may do in the case of humans, for the simple reason that it does not have either.

Our first failed attempt at identifying the particular palm tree involved what is called the Physical Appearance Criterion of personal identity, which states that sameness of appearance is necessary and sufficient for numerical identity (that is, being one and the same). Two palm trees may look identical, but we do not say they are numerically identical. The second criterion of personal identity is called the Bodily Criterion. Applied to our example of a palm tree, it involves the palm tree having a unique body in order to be identified as a particular tree. Other criteria that do not apply for trees but have been argued to be important for personal identity in humans include the Soul Criterion (based on an immaterial soul) and the Memory Criterion (based on remembered experiences).

Richard Swinburne, in his book Personal Identity, presents a scenario where a man wakes up to find himself unable to control the right side of his body. When he would try to move his left-side, he would move his right-side and when he would try to move his right side, his wife’s right side would move instead. This might culminate in the man’s control being shifted to his wife’s body and his wife losing control of hers. Swinburne also considers out of body experiences, where a person “finds himself able to operate on and learn about the world within a small finite region, without having to use one particular chunk of matter for this purpose.”

What do these cases tell us about personal identity? According to Swinburne, although the scenarios may or may not be possible, a person does not need a body made of certain matter nor apparent memories in order to be a person. This is because, Swinburne argues, “all that a person needs to be a person are certain mental capacities — for having conscious experiences (e.g., thoughts and sensations) and performing intentional actions.” Swinburne further argues that personal identity is unanalyzable, that it is impossible to provide criteria that identify persons.

Since neither the body nor the apparent memories are essential to being an individual, Swinburne resorts to a version of a simple theory, called classical dualism, that describes a person as having two parts: a material body and an immaterial soul. He defines the soul as “the essential part of a person,” and states that the continuity of the person is dependent on the soul’s continuity. The fact that it is logically possible for the soul to be linked to a body and separate from it and exist in either a disembodied state or linked to a body makes this theory compatible with the two scenarios discussed above.

In his book Reasons and Persons Derek Parfit presents a thought experiment in which a person is Teletransported from Earth to Mars. The Teletransporter consists of two components: a scanner on Earth that destroys the brain and body of the person while at the same time recording the exact states of all cells and a replicator on Mars that uses new matter to build a brain and body that is exactly like the one destroyed on Earth.

The traveller used the Teletransporter for several years, until a new scanner was introduced that worked slightly differently. This new scanner recorded the traveller’s blueprint without destroying his brain and body. Soon, the scanned person would find out that his Replica exists on Mars and that he, the Earth-dweller, will die of cardiac failure within the next few days. He was then contacted by his Replica on Mars, who, knowing that his counterpart on Earth would soon die, tried to console him and assured him that he would take care of his life.

We may be convinced by a Reductionist View at the intellectual level, a belief that the traveller’s continued existence only involves physical and psychological continuity. However, we may still be inclined to believe in a Non-Reductionist View at some lower level and harbor some fear toward Teleportation, believing that there must always be a real difference between some future person’s being the traveller on Earth and his being someone else.

Our intuitive belief would be that the Replica on Mars is not identical to the person on Earth because, as Parfit writes, we may feel that he would not be him “in a specially intimate way in which no future person will ever be [him].” Nonetheless, Parfit argues that, despite our search for a deeper fact that ascertains our continued existence, ordinary survival is about as bad as Teleportation. He states that personal identity is not what really matters, and thus any doubt or fear in endorsing a Reductionist View is irrational.

Bernard Williams, in his paper The Self and The Future, shows how our intuitions constrain us when formulating a general theory of personal identity. Particularly, the manner in which the cases are presented to us tends to affect our judgment.

Williams presents a scenario where two persons, A and B, exchange bodies after being subjected to a process. It was then announced that of the two resultant persons, the A-body-person and the B-body-person, one will be given 100,000 USD while the other will be tortured. By analyzing how each person will go about choosing the outcome, the case shows that caring about the body a person now has does not necessarily equate to caring about what happens to the person in the future. More importantly, however, Williams’ paper shows how the two persons might have chosen the outcome differently if they were presented with a step-by-step dissection of the process of “exchanging bodies.”

Whether we define our identity by our bodies or memories of our past experiences or something else, such as the soul, the topic of personal identity remains illusory. Despite the reassurance we might get by looking at ourselves in the mirror and reaffirming our identity, a closer inspection of how we confirmed it and continued living our lives is set to reveal a plethora of doubts and paradoxes. Perhaps, as Parfit suggests, we are better off not caring too much about it.