In terms of cause and effect, we like to think that we are the cause of our actions. So what we need to do is look at this picture and then investigate how this would work in real life, starting out with our regular dualistic assumptions about the self.
How would we carve up the world to account for this picture of self?
It seems obvious that we just posit ourselves as subjects, which we believe are some kind of self sufficient causal entity. This entity directs and influences various processes including thought, action, planning etc.
Given these types of processes, among others, we can divide these up into two different species, namely mental and physical causation.
By mental causation we simply mean that this self sufficient entity directs and controls the thoughts and actions that appear.
By physical causation we are referring to physical processes in the body, for example the firing of nerve fibres causes our muscles to move. In this model we have to account for how mental acts can cause physical effects. In contemporary philosophy this is referred to as the mind-body problem.
The Cartesian Picture
There are various solutions to this problem that have been posited over time. Perhaps one of the more historically famous ones would be substance dualism, which is also known as Cartesian dualism.
This doctrine holds that the mind and the body are two distinct substances. This is where the subjective realm exists as a distinctly separate substance from the objective realm and asserts that we are an incorporeal (non-physical) mind that exercises control and volition over a corporeal (physical) body.
This entails that we have two distinct substances of mind and body and thus it is referred to as dualism. This way of looking at the mind-body problem is perhaps the most pervasive throughout our culture.
Its religious credence alone means that this is the version taught, or should I say indoctrinated, to youngsters in schools. It seems to make sense that we are a soul that interacts with the physical body, and this allows the possibility of surviving bodily death.
The reason why dualism seems to be logical is simply because thoughts do not appear to be the same thing as physical matter. If I were to ask you "Where is a thought located?"
It is not as though you can find it in a similar physical space to that which your TV remote occupies. Whilst this may seem a logical way of tackling the problem, it has been found to be completely incoherent and causes intractable problems.
One of the ways in which it seems nonsensical is in how we theorise causation. This is known as the interactionism problem in contemporary philosophy and it goes like this.
In order for us to account for mental causation, we would have to account for how something non-physical, such as a soul, can interact with physical matter.
Given that the physical world is causally closed, that is to say in order to see physical effects there has to be some kind of physical cause, we are left with a puzzle of trying to account for how something non-physical could even possibly interact with physical matter.
Given that in order to move a physical object we have to apply a force to it, then we could question how on earth a mind could even possibly exert a force upon the body.
In order to propagate this theory of non-physical causation we would have to appeal to an auxiliary theory. This would have to be an account of how non-physical mental states can have physical effects in the world.
For example, telekinesis would be a good example of mental states having an effect on the physical world, as would poltergeists. Unfortunately we are in no position to demonstrate this as yet and until someone steps forward to prove their powers, we can rightly assume that these things are an impossibility (please... don't even mention Uri Geller http://skepdic.com/geller.html ).
As Karl Popper remarked, a theories force is diminished when it has to appeal to an auxiliary theory in order to make itself work. However, this is not to say this alone is grounds for its falsity.
What we can do though, is start to look at the Cartesian picture and see if it remains intelligible in light of our assumptions about a non-physical self, or as Gilbert Ryle coined it: The ghost in the machine.
Read Part II here