 Externalists have attacked this argument at its second premise, the premise that causal powers must be intrinsic properties. Against the argument that causal powers must be intrinsic because causation is local, Burge (1986, 1989) has argued that local causation is entirely compatible with broad individuation. Against the argument that the cross-context test for sameness of causal powers shows that they are intrinsic, Burge (1989) suggests that causal powers are typically identified relative to a normal environment; thus, for example, it would be reasonable to distinguish between a heart and a similar organ whose function is to pump waste even if one of these organs could be successfully surgically replaced by the other. Burge (1986) also argues that actual psychological theories, such as Marr’s theory of vision, do not satisfy internalist constraints. (For criticisms of Burge’s interpretation of Marr, see Segal 1989 and Egan 1991. Burge 2010 is a book-length defense of the claim that perceptual psychology is anti-individualistic.)