PM5330: Philosophy of mind

Philosophy of mind is the branch of philosophy which studies the mind, asking questions such as what is its nature.

Some of the major questions in the philosophy of mind include:

  • Is mind ultimately material or immaterial? The major answers include materialism, dualism and idealism.
  • Is there free will? What is free will?
  • The question of personal identity -- what makes me the same person today as I was yesterday and as I will be tomorrow.


Idealism is the theory in the philosophy of mind that holds that mind is the ultimate stuff of reality, and matter depends on mind for its existence. It is opposed to materialism, which hold that matter is the ultimate stuff of reality, and mind depends on matter for its existence; and dualism, which holds that mind and matter are both separate and independent existents, and neither is reducible to the other.

Adherents of idealism include George Berkeley and J. M. E. McTaggart.

Idealism is frequently associated with the idea that material reality is somehow unreal. Many idealists have indeed believed this — famously, McTaggart claimed that the notion of time was so self-contradictory and incoherent that it must be illusory. But not all idealists agree with labelling material reality as 'unreal' — an alternative idealist view, is that while it does not possess the same ultimate reality, the independent self-existence, which mind possesses, it nonetheless is real for what it is.

Idealism was the predominant school in British philosophy at the beginning of the twentieth century; but it swiftly declined and was overtaken by logical positivism, among others.


The term 'dualist' refers to any worldview which supposes the world is composed of two fundamental and independent forces — good and evil (such as in Zoroastrianism and Gnosticism); female and male (such as in Wicca); yin and yang (such as in Taoism); etc. However, the remainder of this article concerns just the philosophy of mind definition.

The dualist view of the mind-matter relationship, holds that mind and matter are two separate and independently existing substances, and either can exist without the other. The other two chief views are materialism — matter is the fundamental existent, the existence of mind is dependent upon that of matter, matter can exist without mind but mind cannot exist without matter; and idealism — mind is the fundamental existent, the existence of matter is dependent upon that of mind, mind can exist without matter but matter cannot exist without mind.

Types of dualism

The major issue for dualism, is to explain, if mind and matter are separate substances, neither reducible to the other, then how do they appear to interact with each other? If one is reducible to the other (as in materialism or idealism), then clearly the two can interact, since their interaction can ultimately be reduced to the interaction of the more fundamental one with itself.

Dualists have proposed the following theories of how matter and the soul may interact:

  • interactionism: this affirms that matter and soul really do interact; the problem is to explain how. Descartes thought the interaction happened in the pineal gland of the brain, but neuroscience has not found any evidence of the pineal gland having such a special role in the mind. A modern alternative, is that the soul influences quantum indeterminancy, which in turn influences neuronal events. But again, there is no evidence for this from quantum physics or neuroscience.
  • pre-established harmony: mind and soul never actually interact, they only appear to. God established them both at the beginning of the world, to evolve separately, each in accordance with its own laws — like how two perfect clocks set to keep the same time, will agree on the same time, without ever interacting with each other to maintain this agreement. This was the view of Leibniz.
  • occasionalism: mind and soul never actually interact, they only appear to. Every time matter needs to act on mind (e.g. sense experience), God miraculously intervenes in the world to alter mind to have that sense experience. Every time mind needs to act on matter (e.g. waving one's arm), God miraculously intervenes in the world to alter matter to perform the action desired by mind. So mind and matter never directly interact, but their interaction is mediated by the constant miraculous action of God. This was the view of many mediaeval Islamic theologians, most notably Ghazali.

The major argument against dualism, is that none of the above theories are believable.

Personal identity

The question of personal identity is what makes me the same person today as I was yesterday or will be tomorrow.

There are three chief answers:

  • dualism - there exists an immaterial soul which somehow controls my body. I am the same person as I was yesterday, because the same soul is controlling my body today as it was yesterday.
  • physical continuity - I am my brain, and I am the same person as I was yesterday because my brain today is physically continuous with my brain yesterday. (Note this is about my brain only, not my body in general - as brain transplantation thought experiments will demonstrate)
  • psychological continuity - I am the same person as I was yesterday because my psychology today is continuous with my psychology yesterday.

A deep question, is not just which view above is correct, but how we can determine which one is correct? One viewpoint holds there is some objectively correct criterion of personal identity. However, it is not clear how we can determine the correct one, or if we could ever prove one correct and the other wrong. Another viewpoint holds that our own personal identity is an arbitrary construct, so we can adopt whichever criterion we wish -- it is a question of what conventions to adopt, as opposed to a question of how the world is.

Teleportation thought experiments

In considering this issue, it is useful to consider some thought experiments. One is the example of teleportation to Mars. Suppose, sometime in the future, the technology of interplanetary teleportation is developed. I need to go on a business trip to Mars, and this new technology is very useful for me -- I no longer need to spend months on a spaceship, I can go to Mars today and be back home in time for dinner. So, I step into the teleportation machine, and the machine destructively disassembles my body into its constitutent atoms, all the while recording with extreme accuracy their position and properties and so forth. This data is then transmitted digitally to Mars, which the teleportation unit at the other end uses to assemble new atoms into the same arrangement.

Now, what am I to think of this machine? One interpretation is that it moves me from one planet to another. Another is that it kills me and I cease to exist, and at the other end a new person is created, a replica, with all of my thoughts and feelings and memories. Upon the return journey, this replica will cease to exist, and yet a third new person will be created on the earth, who will then go home to my family, who will likely mistake this third new person for the original me who is now gone forever. But which interpretation is correct?

According to psychological continuity, it is the original me who is on Mars and returns to earth. Although the physical continuity of my brain may be broken, the psychological continuity of my mind, my thoughts, feelings, beliefs, ideas, memories, desires, dispositions, etc., is maintained. Whereas, according to physical continuity, the second interpretation is correct -- I am my brain, so when my brain was physically destroyed, I was destroyed; and when a replica of my brain was created on Mars, that was not me, it was a new person who happened to have false memories of being me. For the dualist, the question is difficult -- when the copy of my body is created on Mars, is my original soul somehow transported from Earth to Mars? Or does the body on Mars get a new soul? It is unclear how one could possibly tell.

But, it is worth noting, that as people used this teleportation technology more and more, they would inevitably incline towards the viewpoints compatible with their surviving teleportation, such as psychlogical rather than physical continuity. If I have been teleported, I will inevitably believe that I am the same person I was before teleportation -- having all these memories of being the pre-teleportation me, my newly created brain would naturally conclude that it was the pre-teleportation me. If a family member, friend, or lover has been teleported, I will naturally assume they are the same person they were before being teleported -- the alternative, that my real friends and family are dead, and have been replaced by clones, is unlikely to be believable by mentally normal people (it is, in fact, identical to the Capgras delusion.) But, is the fact that we will inevitably come to believe in the truth of a particular theory, for psychological reasons, actually constitute proof that it is true? Many would say it does not.

Let us now consider a variant of this thought experiment. My interplanetary business ventures are thriving, I have been teleporting between Earth and Mars several times a week for the last several years. Eventually I decide to relocate my office to Mars, while keeping my residence on Earth -- this becomes commonplace, many people accepting jobs millions of miles away from their residences. Train stations and other mass transit systems are replaced by giant teleportation banks, which teleport thousands of people an hour to anywhere in the colonized solar system. One morning I leave my house, embark on a brisk five minute walk to the nearest teleportation station. I step into the booth, dial up my desired destination, give the command, and wait. Something is wrong, I fall to the floor, in great agony. An alarm begins to sound. Within a few moments, an attendant appears... "We are terribly sorry, there has been an accident!" The attendant explains that the machine has malfunctioned. It has successfully duplicated me to Mars, but it has failed to completely obliterate me here on Earth. However, while it has not obliterated me entirely, it has caused such severe injuries that my body here on Earth will almost certainly die within the hour. The attendant dials up the teleportation booth at the other end, so I can speak to my replica on Mars, and see that they are perfectly fine. We talk a bit, quite perplexed at what has happened. I then feel myself fading from consciousness.

What has happened here? If we take the viewpoint that teleportation kills me and creates a replica at the other end, nothing has happened to me which has not happened countless times before. It is just that this time my process of suicide has been somewhat slower, so I cannot maintain the illusion that I am not actually ceasing to exist but will go on living at the other end. But, as noted, having used teleportation so often, we are no longer at liberty to adopt that view. We must, for our sanity, believe that teleportation is not a process of destruction and replica creation, but of actual transportation of the original. But then how to explain what happened here? The explanation would be that I have been duplicated into two clones, one on Earth about to die, one on Mars with many years to live -- and I am the clone about to die.

How should I feel? Should I be fearful or despair at my impending death? Or should I believe that I will live on in my replica on Mars? That replica, being an exact replica of me, will continue all my goals, projects, etc. But, this centre of awareness which I right now am, it will cease...

From the perspective of psychological continuity, this is entirely possible. If my new brain on Mars can be psychologically continuous with my destroyed brain on Earth, why can't my new brain on Mars be psychologically continuous also with my original brain on Earth when that original brain continues to exist. So, the psychological continutity criterion permits, at least in principle, that I might have not just one future self, but rather multiple simultaneous future selves, something which the physical continuity criterion does not permit. And, this idea of multiple simultaneous future selves, is vastly different to our intuitions.

So, we are in a sense at an impasse -- physical continuity cannot account for our (inevitable) intuition that we surivive teleportation, but psychological continuity implies the possibility of multiple future selves, which is also contrary to our intuitions. Dualism does not fare any better -- in the case of the failed teleportation, was my soul transferred to Mars? or is my soul still on Earth, and a new soul on Mars? But then, was there not a new soul on Mars every other time I teleported successfully? Or, maybe my soul has somehow split into two replica souls, one on Earth (about to die), one on Mars (with likely many years of life ahead of it.) How do dualists even begin to answer these questions?

One answer to this impasse in our intutions, is that our intuitions are incoherent. Our intutitions treat the question "Who am I?" as if it had some real objective answer, but the question of who I am is just one of arbitrary convention, a semantic game, and any answer is as valid as any other. This viewpoint is particularly appealing to materialists -- whichever theory of personal identity is correct makes no difference to the material world; so, if there is nothing beyond the material world, then criteria of personal identity cannot be anything other than arbitrary conventions.

A good book to read which discusses some of these issues is Derek Parfit's Reasons and Persons (from which, roughly speaking, the above Mars teleportation example was taken).