Yet another solution to the problem was presented by the German philosopher Gottfried Wilhelm Leibniz (1646 – 1716) who rejected interactionism, occasionalism and double-aspect theory in favour of the view that mind and body exist in pre-established harmony. From this viewpoint, no interaction or causation is necessary because, like two clocks that keep the same time, the behaviour of the two substances has been synchronised.

However, pre-established harmony seems to deny the possibility of freewill, though this was a general problem with Leibniz's philosophy. The problem is this: if substances are co-ordinated in pre-established harmony, how can this be compatible with free choice? Leibniz's answer is to point out that, regardless of the actual choice made, another choice is possible. So, although it might seem to be predetermined that you are reading this, it was logically possible that you might have done something else.

This is not really a very convincing argument, for to accept that we have a choice we would have to consider it a contingent truth that something happens (i.e. it could have been otherwise). However, in organising the pre-established harmony God has chosen the best of all possible worlds. He has done this, obviously, because he is good - necessarily good, that is. So, doesn't this then mean that God must choose this world and everything is predetermined?