 The debate here quickly gets into technical arguments about the concept of causation. Campbell argues against Frith in favour of the Humean idea that we simply cannot tell in advance of inquiry what causal relations obtain in nature. Anything can cause anything and it is just as unwarranted to suppose that the relevant explanatory variables for psychotic or affective phenomena must be cognitive as it is to adopt a simple reductionism and insist on their being biological. Decades of research have revealed that mental illness is caused by, and causes in turn, many different physical, psychological and social factors. We simply have to take our causal relations where we find them, including interlevel ones. Craver and Bechtel (2007, 554) on the other hand, argue that it stretches the concept of causation to breaking point to admit interlevel causes: they say that “to accept interlevel relationships as causal violates many of the central ideas associated with the concept of causation”. Craver and Bechtel argue from the perspective of mechanistic explanation. Mechanisms explain effects in terms of interlocking parts, and the relation across levels, they affirm, is one of constitution, not causation; causation can only be intra-level. Interlevel causation, on this view, amounts to something causing itself, because different levels are different ways of talking about the same thing. However, talk of levels in psychiatry often involves relating different parts of the world, as in Meehl’s example: genetics and bereavement may be seen as requiring different levels, but they are not different ways of describing the same process. Causal models must interrelate phenomena of very different types.