In this essay, I shall defend a transcendental argument for epistemological realism: the view that mind-independent yet cognitively accessible entities exist. The proposed argument reasons from the fact that we are conceptual creatures to the existence of a knowable outer world as a condition of the possibility of such creatures. I first lay down my general approach to concepts and conceptualization, according to which concepts are rules that agents follow in their cognitive activities. I go on to explicate the peculiar (...) normative nature of rules and rule-following, from which I extract, following Wittgensteinian considerations, an intractable problem for any idealist account of concept possession. I argue that the very possibility of conceptualizing requires the existence of external objects that enable the cognizer to regulate their use of concepts, and close with some remarks on the resultant nature and scope of metaphysical knowledge. (shrink)