Naturalists who conceive of knowledge as a natural kind are led to treat ‘knowledge’ as a natural kind term. ‘Knowledge,’ then, must behave semantically in the ways that seem to support a direct reference theory for other natural kind terms. A direct reference theory for ‘knowledge,’ however, appears to leave open too many possibilities about the identity of knowledge. Intuitively, states of belief count as knowledge only if they meet epistemic criteria, not merely if they bear a causal/historical relation to the term. I will develop this objection and show that it is grounded in modal considerations central to Kripke’s work on reference. I will also argue that a more plausible externalist semantics for natural kind terms disarms the objection.