Advocates of extended mind often appeal to the theory of mind known as ''functionalism'' in support of the idea that the realization of mental properties can extend beyond the brain. In this paper I argue that functionalism is largely silent with respect to the extent of a realization. This is because functionalism describes only those functional relationships that define a mental state: functionalism does not have the means by which to distinguish the realization of a mental state from causal influences on the realization. Because functionalism does not dictate the boundaries of the mind's realization, the case for extended mind must draw on non-functionalist considerations.