  Schaffner (1993, 1994) suggests this structure is the norm throughout biology, and sees it as relating causes and effects at different levels of explanation. Appealing to levels of explanation is unobjectionable if it just involves a reminder that we need to relate variables of many sorts. But it is not clear that we have any principled grounds for sorting phenomena into levels. Marr did, because he imagined levels as descriptions of the same process (the construction of a 3D image from 2D retinal impacts) couched in the vocabularies of different sciences. But when we move outside the skull and begin introducing environmental factors and other kinds of cause, the Marrian picture looks less plausible.