  Most discussion of interactionism takes place in the context of the assumption that it is incompatible with the world’s being ‘closed under physics’. This is a very natural assumption, but it is not justified if causal overdetermination of behaviour is possible. There could then be a complete physical cause of behaviour, and a mental one. The strongest intuitive objection against overdetermination is clearly stated by Mills (1996: 112), who is himself a defender of overdetermination.