 In part because of these issues, that levels in psychiatry are not Marrian, Campbell (2006) argues for an interventionist approach to causation. This is the view that when we say X is a cause of Y we are saying that intervening on X is a way of intervening on Y (Woodward and Hitchcock 2003, Woodward 2003, Pearl 2000): manipulating one variable makes a difference to another. This is not a reductive analysis of causation, since it makes use of causal ideas—it just states that questions about whether X causes Y are questions about what would happen to Y if there were an intervention on X. Kendler and Campbell (2009) have argued that an interventionist model provides a rigorous way of articulating the idea that any combination of variables might characterize the causes of a disorder, whilst at the same time providing a clear test of what variables are actually involved, thus avoiding a simple-minded holism that just says that lots of things are relevant. They further note (2009, 997) that interventionism “permits the clear separation of causal effects from the mechanistic instantiations of those effects”, thus directly confuting the approach favored by Bechtel and Craver.