  Most mental illnesses are caused by diverse environmental and genetic factors. We have little reason to think that all the relevant causes can be given a reductive analysis, for as Schaffner dryly points out, to specify and measure environmental variables in purely molecular terms “would be a very long-term project” (1994, 287). But one might still think that molecular processes are the root causes of mental illness even if many other contributory factors exist and define the context within which genes work. Kandel (2005) offers a very stringent version of this view, which assigns all explanatory significance to proximal genetic causes on the grounds that “all of ‘nurture’ is ultimately expressed as ‘nature’ (39)” He means that behavioral and cultural factors influence gene expression, but he also seems to think that this implies that psychiatric explanations need only mention gene expression, which will screen off everything else.