  Marr sought to understand psychological capacities in the abstract without worrying about biology: although he certainly assumed that capacities could be realised in a physical set up, he thought that the details of the particular set up would not make a difference to the way the psychology was understood. But as the sciences of the mind have become more interested in the brain we are learning that this idea may need to be amended. Cognitive neuroscience is learning to look for physical structures in the brain that carry out information-processing jobs, or what Glymour (1992) calls “cognitive parts”. Our understanding of biological realization feeds back into and constrains our understanding of the abstract processes of cognition (Bechtel and Richardson 1993). Rather than relating abstract psychological capacities to each other, higher level descriptions of the mind/brain in cognitive neuroscience relate capacities understood as functional descriptions of fairly coarse-grained brain areas, not purely abstract computational task specifications. Theories of the normal system specify the nature and interrelation of cognitive parts. Armed with these assumptions about cognitive parts, psychiatrists can see a disorder as a breakdown in normal functioning within and between cognitive parts. Marr’s first level might seems to be useful as a source of background theory for doing psychiatry, because it asks “what is it for?” and much of psychiatry is about finding out psychological systems fail to do what they are for.