The mind/body problem that was a concern in the Middle Ages and in early modern times is, however, as indicated not the same problem that occupy contemporary philosophers. Today we want to explain how phenomena like consciousness and intentionality are possible in a material (or physical) world. The problem that faced medieval philosophers and Descartes was rather the opposite, that is, how can matter at all have an effect on the mental (non-material) and how can such a noble thing as a mind be united to a material body.
- One problem is the so-called interaction problem, that is, how can such different things (or substances) as the mind and the body have an efficient causal effect on each other.
- Another problem is the unification problem, that is, how can the mind and the body, which can exist apart from each other, be united into one single thing; a human being.
- A third way of stating the problem has to do with the existence of sensations or sense ideas in the mind, which means that the problem is really how to explain in what way there can be sensations in a mind without a body.
- A fourth mind/body problem, which is quite neglected and which the present book does not deal with at all, but which is very important, is how final and efficient causality can be combined.