English philosopher Thomas Hobbes argued that we can only form ideas of physical things. This means that if we do have a soul, we cannot become aware of its presence.
Hobbes claimed that:
"the soul is something of which we have no idea at all. We rationally infer that there is something within the human body which gives it the animal motion by means of which it has sensations and moves; and we call this 'something' a soul, with out having any idea of it".
Hobbes' contemporary, French natural philosopher Rene Descartes, took a different view, and suggested that the soul may be the only thing we can be aware of.
Descartes was also a mind-body dualist; because he could conceive of his mind existing without his body, he concluded that the mind must be made of an entirely different substance, a substance that thinks.
In Meditations on First Philosophy, first published in 1641, Descartes suggested that the mind differs from physical substances in three ways:
The mind experiences sensations that cannot be explained mechanically.
The mind does not exist in physical space like the brain does.
The mind is a necessary whole, it cannot be divided or replicated in the same way that a physical object can.
Descartes' conception of the mind also differs from physical matter because there is currently no room for subjectivity in physics.
Any physical theory of the mind will have to solve these four problems.
1.1 Problem 1: Explaining qualia ↑
Descartes believed that phenomenal qualities, like colours and sounds, must be made of a different substance to physical matter because they do not exist within our scientific description of the world.
We perceive different wavelengths of light as different colours, for example, but there's nothing within physics that explains why interactions with light should result in the experience of a colour, or why we associate any particular colour with any particular wavelength.
Wavelengths corresponding to coloured qualia. Image credit: NASA/Public domain.
Colour could be replaced by any of our sensory experiences to illustrate this problem: sound is a physically indescribable representation of the vibrations of air molecules, touch and pain represent signals sent from nerve cells, and taste and smell represent the experiences of different chemical reactions. These sensory experiences, as well as other subjective internal experiences, like emotions, are described as qualia.
Qualia are defined by the fact that you cannot know what they are like without experiencing them for yourself, and you cannot compare qualia with other people. This is evident from the fact that someone who has been blind since birth cannot imagine a colour they have never seen, even if they have a complete understanding of the physics of light.
If we take sensory, or phenomenal, experiences as our only guide to reality, then we will be led to the assumption that the physical world does not exist.
If, on the other hand, we take physical science as our guide to reality, then we will come to deny that phenomenal experiences exist.
Descartes chose the former and argued that common sense realism, the view that we are directly acquainted with physical objects, is false. We cannot directly perceive the external world because when we look at an object we are only aware of the phenomenal qualities that fill our sensory fields, and these can exist even when there's no object present. This can happen when we dream or imagine, for example.
Descartes claimed that the same is true of emotional responses and even pain, using the phenomenon of phantom limbs as an example of pain that does not correspond to the physical body. Since we can experience qualia without the need for external objects, Descartes claimed that there's no reason to believe that qualia correspond to external objects at all.
Descartes stated that "there is never any reliable way of distinguishing being awake from being asleep"[2a]. This is known as the dream argument.
Descartes claimed the dream argument shows that knowledge comes from rational introspection. He stated that:
"when the mind understands, it somehow turns in on itself and inspects one of its own ideas; but when it imagines, it turns away from itself and looks at something in the body (something that conforms to an idea - either one understood by the mind or one perceived by the senses)"[2b].
Descartes illustrated the difference between introspection and the imagination with the following example:
"When I imagine a triangle, for example, I don't merely understand that it is a threesided figure, but I also see the three lines with my mind's eye as if they were present to me; that is what imagining is. But if I think of a chiliagon, although I understand quite well that it is a figure with a thousand sides, I don't imagine the thousand sides or see them as if they were present to me"[2c].
Like Plato, Descartes believed that mathematical concepts contain an element of truth that goes beyond what we are capable of imagining. This led Descartes to claim that sciences like physics, astronomy, and medicine are "doubtful" yet mathematics is "certain and indubitable". Descartes stated that "whether I am awake or asleep, two plus three makes five, and a square has only four sides"[2d].
Despite the conclusions of the dream argument, Descartes went on to claim that rational introspection could also be flawed. In order to demonstrate this, he invoked the idea of an all-powerful God and asked:
"How do I know that [God] hasn't brought it about that there is no earth, no sky, nothing that takes up space, no shape, no size, no place, while making sure that all these things appear to me to exist?"[2e].
Descartes concluded that God could have the power to deceive us about mathematics and other rational knowledge, and so we can't even be certain of this. Descartes supported the idea that God could not be deceptive with the argument that: "if God's goodness would stop him from letting me be deceived all the time, you would expect it to stop him from allowing me to be deceived even occasionally; yet clearly I sometimes am deceived". For those who would not accept a deceiving God, Descartes extended the theory to include "some malicious, powerful, cunning demon"[2f].
Descartes realised that the only thing he could be certain of was his own existence. Although he could be tricked into thinking anything, he could not be tricked into doubting that he is thinking at all, and since there are thoughts, there must be existence.
"I shall consider myself as having no hands or eyes, or flesh, or blood or senses, but as having falsely believed that I had all these things. I shall stubbornly persist in this train of thought; and even if I can't learn any truth, I shall at least do what I can do, which is to be on my guard against accepting any falsehoods, so that the deceiver - however powerful and cunning [they] may be - will be unable to affect me in the slightest. This will be hard work, though, and a kind of laziness pulls me back into my old ways. Like a prisoner who dreams that [they are] free, starts to suspect that it is merely a dream, and wants to go on dreaming rather than waking up"[2g].
1.2 Problem 2: Where is my mind? ↑
Descartes used the problem of qualia to show that the mind must be composed of a different substance to physical matter. He also claimed that the mind differs from physical substances because it does not exist in space. He believed this to be evident from the fact that he could imagine his mind existing without the need for a body.
Descartes stated that:
"I have a vivid and clear idea of myself as something that thinks and isn't extended, and one of body as something that is extended and does not think. So it is certain that I am really distinct from my body and can exist without it"[2h].
This argument could be contested by the fact that we have no proof that the mind can exist apart from the body. Even if we can imagine such a thing, it does not make it true since Descartes showed that we can be tricked into these beliefs. But the problem becomes more difficult when we try to give a location to the qualia we experience in our imaginations.
We can relate different experiences to different locations within the brain but we cannot show that qualia reside there without first assuming that they do. We have yet to find a part of the brain that is coloured red, that sounds high-pitched, or smells of cinnamon, for example.
1.3 Problem 3: The unity of consciousness ↑
Descartes extended the idea that the mind does not exist in space to show that the mind cannot be divided into parts.
Descartes claimed that it's impossible to conceive of our mind being halved or duplicated. We can imagine losing all sensation in half of our physical body, losing half of our memories, or half of our brain functions. We can even imagine going insane so that our thoughts are not coherent, but we cannot imagine seeing half a colour, or seeing two different colours in the same space at the same time.
Descartes argued that as long as it is thinking, the mind is always experienced as being a single, and complete, entity. He stated that:
"the mind can't be divided. When I consider the mind - i.e. consider myself purely as a thinking thing - I can't detect any parts within myself; I understand myself to be something single and complete...it is one and the same mind that wills, understands and perceives"[2i].
The unity of physical objects, on the other hand, always occurs as a matter of degree with a seemingly infinite amount of possible subdivisions.
Descartes stated that:
"any corporeal thing can easily be divided into parts in my thought; and this shows me that it is really divisible"[2j].
1.4 Problem 4: Subjectivity ↑
Finally, the nature of qualia reveals the inherent subjectivity of the mind. There's no way to compare the qualia we experience with others and so no way to know if others really experience the same sensations we do.
This makes the mind different from the substances dealt with in the physical sciences because they only deal in facts that can be proven true or false, irrespective of human opinion.
This means that any physical theory of the mind will have to differ greatly from traditional science, not only will it have to reference subjective opinion, but it will have to explain why we have subjective experiences at all.
2. Problems with Descartes' theory ↑
Descartes showed that any physical theory of the mind will have to explain why we experience thoughts of any kind, including qualia and rational introspection. It will also have to explain why qualia cannot be communicated with others, where qualia reside within the brain, and why we cannot divide these thoughts into parts.
Descartes solved these problems with dualism, the idea that the mind is composed of an indivisible, thinking substance that does not exist in physical space. This solution is not without problems, however. The biggest problem is that Descartes did not explain how such distinct substances could interact.
It's clear that the mind must have some control over the body; this is evident when we decide to move. The body, in turn, has some control over our mind; this is evident when we feel pain. The problem of how the mind and body interact, despite obeying different physical laws, is known as the problem of causal interaction.