The problem with Descartes' definition of mind and body is that
he seems to be describing two mutually exclusive substances. If,
in every respect, matter differs from mind, how are the two meant
to interact? Descartes needs to explain this because, unlike other
theories which we will look at later, there is meant to be a causal
interaction between the two substances – in other words, mind
influences matter and vice versa. This is called interactionism.
However, the difficulty here is how a material thing – our
body – can be influenced by a non-material thing – our
mind. By Descartes’ definition mind is immaterial, having
no dimension, mass, etc. But then how can something without any
physical effects influence a physical thing? According to current
scientific thinking, physical objects are only affected by physical
forces. Even if we think in terms of atoms, molecules or particles,
we are still talking about things with dimension and mass –
however small. In this sense, the material world is a closed circuit
of cause and effect.
Descartes’ response was to suggest that the
two substances meet in a part of the brain called the pineal gland.
His reasons for choosing this seem to have been that the gland in
central (unlike the other parts of the brain which are bilateral
– mirrored on each side) and that it does not occur in animals.
This latter fact was understood by Descartes as relating to the
presence of a soul in humans and not in animals, whom he considered
mere machines. However, modern research has also found a similar
gland in mammals and lower vertebrates.