The reason philosophy of mind exists, is because the physical evidence generally does not support the view known as dualism. Dualism is the position that we have "spirits" or "souls" and our minds are not somehow dependent on our bodies; that is, that there are two types of existence; mental/spiritual, and physical/material. Most of the work in this area of philosophy is an attempt to account for why it feels like dualism is true, when the evidence seems to show that it is not.
Target: Dualism
1. Supervenience
2. Epiphenomenalism
3. Functionalism
4. Eliminativism
5. Central state materialism
6. Extended mind theory
7. Panpsychism
8. Emergence
9. Temporal model
I last studied this material around 2005 so I may be rusty and mis-remember authors and how to present the cases, but here's a summary. If I've left any out, please contact me on social media and advise. Any material mistakes or corrections welcome.
Target: Dualism
David Chalmers (older work) and Rene Descartes. This is the default theistic or spiritualist position. It denies the materialist premise that minds are created by or caused brains. It claims that there is a separate mind (or soul, or spirit), which correlates with or follows the body. It appeals to our intuition that there's something special about the mind and its unique "incorrigible" first-person perspective, that is, that we have access to our private thoughts and experiences. However, there are a number of problems with the position which strongly hint that it's not correct.
Firstly as mentioned, if the brain itself is struck, we lose consciousness. This suggests that it is tied to the head, not a disembodied soul.
Secondly, commissurotomy studies demonstrate split consciousness (Marks 1981 disputes this but I believe he fails to make his case). Commissurotomies were performed in more barbaric times so as to control epilepsy. If the commissures (joining area between the two sides of the brain) are cut in a commisurotomy operation, the two halves start behaving like separate people. For example, one hand pulls up the trousers, the other pushes down. One hand writes something under a concealing surface, the other cannot draw a picture of what was written. Yet both sides are individually aware of what they did, and neither side is aware of what the other side did. There are also experiments which show that we are not aware of our brain activity anyway, e.g. Soon et al (2008) show that our brain "Decides" what to do about 10sec before we actually do it, yet we're only "aware" of the decision about 300msec before we move.
Thirdly, it's trivial to show that our access to our mind's contents is not incorrigible (faultless) and in fact it is very poor. For proof of this, google "visual illusions", "Elizabeth Loftus memory reliability", "blind spot", "cocktail party effect". You'll find many other related cases where in fact it can be shown that we're mostly aware of what we're looking at, or focusing on, and only for a short period of time. Optical illusions show how our brain makes up content. We are actually barely conscious at all. For example, this illusion (Hermann Grid illusion) shows grey blobs in the white intersections. There ARE no grey blobs. Also, in one of the variants, the blobs seem to shimmer and appear/disappear.
Fourth, the causal efficacy problem. Descartes was a famous case of a dualist in the scientific era. His problem is the well-known pineal gland problem. He discussed the nature of the mind and concluded that it must have a way to control the body. He proposed the pineal gland as the interchange point between the spiritual and physical. However there's no modern scientific evidence or reason to support that choice, or to explain why that part of the brain only, can transmute spiritual events into physical world events. So this is the problem of causal efficacy of the spiritual.
If you commit to the view that the spiritual is causally efficacious, you commit to the view that the spiritual just is physical. Maybe it's a field, or particles, but it's physical, because it manifests effects in the physical world through presumably contact. The same argument can be extended to any spiritual entities (ghosts, demons, deities, etc). Either they're not able to have any effects (do not exist, even?), or they are physical (and therefore testable by science).
Chalmers used to support dualism, as I interpret him, and I do not believe he was successful. He now seems to have migrated to Clarke's view, below.
1. Supervenience: the mind somehow "floats" or "supervenes" or "sits" on the brain. More like a field. I believe John Searle (2001 articles) would hold this position. The trouble with the position is it seems to be quasi-dualism; like it wants to separate the mind (to give it its magical first-person perspective properties and subjective properties) but still make it physical. However this does not seem to explain how the physicality of the mind makes consciousness possible, nor provide a route for us to hope that consciousness could be causally efficacious (ie that we control our actions at all). It just allows that the mind is physical and the brain is physical, without saying how the mind is generated and whether it can do anything other than represent "seemings" to "us".
2. Epiphenomenalism: Author: Daniel Dennett. The mind is a side-effect of the brain's activity, like a shadow is a side-effect of light shining on a body. It is visible, it exists, but it has no significant effects. Dennett argues that our belief in our own causal efficacy is little more than that; a belief. The actual bodily movements we make are controlled mechanically by brain processes. Dennett skirts dangerously close to eliminativism at some stage when he disparages mental "seemings" (e.g. that it seems to me that I now see pink), as "qualia" and "figment" (a play on figment of your imagination and "pigment"). I am sympathetic to this view, because it explains certain anomalies in how we can respond fast to physical threats but only become aware of them consciously afterwards.
The main trouble with the position is that if the mind is not causally efficacious, that is, it's just a shadow-like effect, why does it exist, from an evolutionary standpoint? Prima facie it seems that the purpose of the mind, from an evolutionary standpoint, would be to keep us alive and safe from holes in the ground, predators, etc. However if the mind is not causally efficacious, why have it? I suspect the answer is it might be an anachronism or similar; something like an appendix or tonsils. It's there because having it hasn't killed us all off.
3. Functionalism: Author: David Lewis (Article: Mad Pain and Martian Pain). The mind is caused by the brain, and the physical substrate itself doesn't matter as long as it's functionally equivalent. So if a Martian has a hydraulic brain rather than an electrical one, we won't deny him consciousness even though his brain is different.
For AI to be possible in the true sense - that is, conscious robots - we'd need functionalism to be true, since robots' minds are implemented with doped silicon and metal circuits. Not neurons. That means that if functionalism is false, robots will never be conscious unless they're made of cells, neurons, etc.
The problem is best explained as the emulation-vs-instantiation problem: Even if you emulate the behaviour of neurons in hardware or software, you're still not instantiating a mind, you are emulating or copying one.
I am sympathetic to functionalism because I think if we're committed to the scientific world view - that ultimately everything is just matter and interactions between particles of matter, forces, etc., - then necessarily functionalism follows. The position is however not really able to explain why human brains have the qualitative experience of consciousness in the first place, because it just makes it possible for any suitable system to have consciousness. In short, functionalism is not discriminating enough in what it would count as conscious. It doesn't say how it's caused or why it feels like that. To parody it, a jersey is complex and has patterns and networks and threads. But we do not think jerseys are conscious.
4. Eliminativism: Author: Patricia Churchland. Position: There's no such thing as a mind. There's only the brain and its activities. The mind - consciousness - is eliminated. I cannot recall the argument for this position other than to say it is a strong form of central state materialism (below). I have presented it very poorly here. However I do not see we should entertain it.
I think it's clear that this position is just throwing the problem out rather than answering it. We all do very clearly know that we feel conscious. Even if eliminativism were true, that there's no such thing as a mind, it still doesn't explain this experiencing now, or for example why we feel like a colour-blind person really does have a different experience of the world to a non-colour-blind person. There's a thought experiment called "Mary the scientist". It goes as follows. Mary grows up in an all-white room where everything shown to her is shades of grey. She never has a mirror. When she goes outside finally into the outside world and sees coloured objects for the first time, does she learn anything? All of us, I think, would say yes, she would be startled. Churchland however has to bite the bullet and say no. But I seriously doubt it - Mary would definitely notice. It can be empirically demonstrated. Give colour-blind correction glasses to a colour-blind person and watch how dramatically they respond even though they are generally only unable to see half of the colours. They definitely learn something. Now imagine someone who has never seen colour ever! They'd be overwhelmed. You can also find videos of deaf children achieving hearing for the first time. Their astonishment is visible. To me this is a demonstration that eliminativism is false.
5. Central state materialism: The mind is just the activities of the brain or central nervous system (CNS). Author: DM Armstrong (1968). This is the default position in science, I think, but it doesn't answer the question of how the brain produces the mind, because computers are very brain-like but give no sign of having minds of their own (well, except when they crash or play other pranks on you).
The simple and obvious explanation of why CSM must (largely) be true comes from the Ancient Greeks: if you drop a block or object on your toe, your toe feels painful. If you drop the same block on your head, you are knocked unconscious. Therefore, consciousness is inside your skull. The issue of why we feel sensations e.g. in our limbs is well-explained in terms of how the mind maps the body in space. Hence the phenomenon of "phantom limbs" in amputees who still "feel" their limb even though there is no limb. Similarly, it is very clear that the neural density of the body is maximal at the brain, and if nerves are severed, then consciousness of that extremity is severed (well except when people get phantom limb illusions. For more on that, google "false hand illusion"). So we know that consciousness has something to do with neurons.
Similarly, when people are placed under fMRI, we are able to see changes in the energy use in the brain at various locations depending on the mental task used. The Japanese have even been able to extract images from the brain.
The science in this area is quite advanced now. They can read pictures, words, and music, from the mind, just using fMRI and an AI that inteprets the scan. The accuracy is over 80% in most cases.
So consciousness resides in the brain; that much is conclusive. Whether it depends on the brain, or whether it is limited to just the brain, or whether it can be instantiated elsewhere, are open questions.
The main problem with CSM is it doesn't say how consciousness is made.
6. Extended mind theory: This is Andy Clarke's position and it's a relatively new one (this century; I first encountered it in 2008). It supposes that the mind is extended - not just the brain, but extended to things like our social circles, memory storage methods, the environment, etc. More like a hive mind. It is characterised as an "active externalism", meaning the mind isn't just in the skull.
I do not see that it solves the problem because it doesn't account for the first-person perspective, even if it's true. And I find it problematic to make the claim simply because of the existence of individual consciousness. While I am sympathetic to the idea of a hive mind - like we see in say, termite colonies - it's not really one mind with one point of view; it's just coordinated activity. I recognise however that as Dennett and Eugene Marais argue, our minds are a lot like termite colonies: millions of small agents working together to create a metaphenomenon or emergent phenomenon we call the mind. However, I suspect that extending it to the outside world is stretching the truth. I am sure I am parodying the position, however, so see the below videos for more detail.
7. Panpsychism. This position is that everything has a mind - even particles of matter. There is something-it-is-like-to-be an electron. And so, we should not be surprised that brains have minds. A more nuanced defence of a similar position is available in my paper on Academia.edu:
I also explain why it's not useful. The pantheism model explained in the paper relies on a version of panpsychism, so don't be distracted by that difference. Pantheism is the view, roughly, that the universe is God. Panpsychism is the view that everything has a soul or mind. So, these two theories may well be coextensive if God is just an infinitely large soul. (To put it more pedantically: If God is an infinite soul - pantheism - and if the universe is infinite and all things are conscious - panpsychism - then panpsychism and pantheism have the same extension - thing they point to - even if they have a different intension or meaning).
The trouble with panpsychism is that it just makes consciousness an unexplained phenomenon or a property of matter, like "charge" or "mass" or "extension". It also creates further problems, e.g. with ethics. It means that, for example, we can't mine minerals anymore because we are literally hurting the earth by so doing. But the most implausbile thing about panpsychism is that it is just "kicking the can down the road." If our neurons are conscious because electrons are conscious, that doesn't explain why electrons are conscious. It's still not answering the question except to offer us a brute fact (matter just is conscious).
The main advantage to panpsychism is you would now have good reason to swear at the coffee table when you stub your toe.
8. Emergence. I suspect the correct answer to the problem of consciousness lies in a combination of epiphenomenalism, functionalism and emergence theory.
We want a model which creates consciousness out of specific arrangements of forces, energy, matter, etc., just like life is made out of, or comprises, DNA, cells, osmosis, energy transfer, mitosis, meiosis, etc. all of which are mere physical/mechanical processses. So we want a model of emergence; that is how a certain arrangement of matter/energy gives rise to consciousness; in the same way that traffic emerges from cars, or ocean currents and whirlpools emerge from water molecules, or fields of energy emerge from many energetic particles. This is not to say the mind is not real, but that it is indeed a kind of summative effect or consequent effect of subsidiary processes.
Let's return to question of 'what is life', so as to provide an analogy. People debate whether viruses count as "alive". Viruses reproduce using other cells, but they do not have respiration or combustion, and they comprise only a cell wall and DNA/RNA (excuse my poor definition). They're more similar to organelles inside living cells - such as mitochondria - than living cells. So it's clear that there are certain threshold conditions of complexity for something to be considered alive.
Inasmuch as we are not clear whether a virus could be alive, or whether a mitochondrion can be considered alive, or whether a bacterium is really alive, we might ask the same about consciousness, using the same kind of grading scale. In virtue of what do we get consciousness, and how many neurons do we need? Is a jellyfish conscious? A tapeworm? A bacterium? What about an amoeba? A starfish? A lizard? A spider? Etc.
Let's put it this way. To explain living things, at some point, you have to talk about non-living components, systems, and energy flows. Otherwise you are forced to kick the can down the road and say all atoms and quarks and electrons have elan vital. The same for consciousness. Either we explain it by reference to non-conscious mechanical parts, or, kick the can, all particles are conscious and consciousness is a property like magnetism, electrical charge, etc.
I think the matter relates to (a) complexity and (b) neurons, nothing more.
But even if we succeed in making a robot with a brain which operates exactly like ours (except made say of silicon), we can at most say that the robot looks conscious and says it is conscious and says is is having conscious experiences, but we can never know; it could just be lying or programmed to say so. As Nagel observes: we do not know what it is like to be a bat, and feel the experience of using the sense of echolocation. So we would not know what it was like to be a robot.
9. Temporal model. A temporal model of the mind would argue that the mind either just is the same as time, or, it exists in time only (4th dimension), or it is supradimensional (e.g. 5th dimension). I've heard this view from a few people but I've not thought about it enough to give it either a positive or negative response. In some sense, the experiencing of time is fundamental to our consciousness (e.g. how a dream seems to be a few minutes but actually in real time is an hour). And our awareness of time passing is something central to what we consider consciousness. However, whether the mind is coextensive with time, or coextensive with another dimension, or whether it merely accesses another dimension, is interesting but speculative and would need some significant argument.
Disclaimer
Please note that all the above positions are strongly defended by many competent authors, most of whom hold professorships. The articulation I have presented above does not do these positions justice as to how nuanced they are.
My position
I am not firmly convinced by any of the above but I think a combination of emergence, epiphenomenalism, CSM, and functionalism, must be approximately right.
