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.