I. The Mind-Body Problem
There is an age-old problem in philosophy known as the "mind-body problem." One quick way to state the problem is this: what is the relationship between the mind and the body -- between the mental realm (the realm of thoughts, beliefs, pains, sensations, emotions) and the physical realm (matter, atoms, neurons).
Are your thoughts, feelings, perceptions, sensations, and wishes things that happen in addition to all the physical processes in your brain, or are they themselves just some of those physical processes?
This should be interesting to you for a couple of reasons:
(1) It's just an interesting philosophical puzzle -- belief in an immaterial soul seems kinda wacky, but on the other hand it seems sort of mysterious how a physical system like a brain could give rise to mental states.
(2) An answer to the debate between materialism and dualism is an answer to the question, What am I? If materialism is true, then you are a physical object -- an organism. If dualism is true, then you are a soul (or maybe it's better to say that you are a composite object-part soul, part body).
A. Cartesian Dualism
Cartesian Dualism is the conjunction of the following five theses (I have underlined the technical terms and defined and discussed them below):
CD: (a) Each person is composed of two main parts: an immaterial mind and a physical body.
(b) Only immaterial minds can have mental properties.
(c) Only physical objects can have physical properties.
(d) Mind and body are able to exist independently (and generally do so after death).
(e) Mind and body enter into two-way causal interaction.
immaterial mind =df. a spatially unextended thinking thing
There are two ways something might be spatially unextended: (i) it might not exist in space at all, and so the notion of spatial extension doesn't apply at all; or (ii) it exists in space but it is so tiny as to be absolutely dimensionless -- it has no length, width, or height; it is like a geometrical point.
It is not clear whether Descartes thinks minds exist inside space or outside space, so our interpretation of Descartes' dualism will remain neutral about it.
mental property =df. a property such that anything that has it must be conscious
So if a thing has a mental property then it is guaranteed to be conscious.
Examples of mental properties:
being in pain
believing the Red Sox are in first place
tasting the taste of chocolate
wanting the semester to be over
Each of these properties is a mental property because if you have any of them, then you have to be conscious. There is not way to have any of these properties without being conscious.
Examples of properties that are not mental properties:
stubbing your toe
saying, "Woo-hoo, the Red Sox are in first place"
saying, "Damn, I can't wait for the semester to be over"
None of these properties is a mental property because it is at least logically possible to imagine a thing having any one of them without being conscious. For instance, we could build a robot that had all of these properties but had no mental states at all. It is easy to mistake the above properties for mental properties because, as things stand in our world, these properties are tightly causally connected with mental properties.
physical object =df. a thing that is extended in space and time
To say that a thing has extension -- that it is extended in space -- is to say that it has bulk. That it takes up space. That it takes up room in the universe.
Examples of physical objects: my body, your body, Lassie the dog, the chair I am sitting in, the Eiffel Tower, a hydrogen atom, the planet Earth, The Milky Way galaxy
An example of an object that is not extended in space and time? This is harder to come by. Even if we squeeze something down really, really tiny, so long as it has just a bit of length, or width, or height, it is still extended, and so is a physical object. The points studied in geometry, if there are such things, are not physical objects, since they are not extended. If there are abstract objects, such as numbers, presumably they are not objects with extension, so they are not physical objects.
physical property =df. a property such that anything that has it must be a physical object
So a physical property guarantees that whatever has it is spatially extended.
Examples of physical properties:
weighing 150 lbs.
being 6 feet tall
stubbing your toe
having a chemical change in your brain
See Sober p. 261, "Immortality of the Soul," for some comments relevant to clause (d) of CD.
About clause (e): Descartes thinks that, although mind a body are two distinct things, they can enter into two-way causal interaction. So, events in the body can cause events in the mind: for example, the stubbing of a toe can cause the firing of a neuron in the brain which can cause the sensation of pain in the mind. Also, events in the mind can cause events in the body: the desire to drink the water can cause the firing of some neuron in the brain which causes the contraction of the arm muscle which causes the raising of the water bottle to take a drink.
MM: (a) Each person is just a physical object (his/her own body); and
(b) There are no souls or immaterial minds; and
(c) Each person (that is, each human body) has mental as well as physical properties.
I call this 'minimal materialism' because it contains theses that any reasonable form of materialism would contain, but it is in important ways incomplete. It is in complete because it does not tell us how it is that a physical object like a human organism can have mental properties. That is, it does not include a theory of mind.