The problem with metaphysical realism, according to Putnam, is that it fails to explain the possibility of reference and truth. According to the metaphysical realist, our concepts and categories refer because they match up in some mysterious manner with the categories, kinds and individuals inherent in the external world. But how is it possible that the world "carves up" into certain structures and categories, the mind carves up the world into its own categories and structures, and the two carvings perfectly coincide? The answer must be that the world does not come pre-structured but that the human mind and its conceptual schemes impose structure on it. In "Reason, Truth, and History", Putnam identified truth with what he termed "idealized rational acceptability." The theory, which owes something to C. S. Peirce, is that a belief is true if it would be accepted by anyone under ideal epistemic conditions.