 Acquaintance is typically taken to be simple and thus indefinable. The philosophers for whom acquaintance plays a critical philosophical role are usually unapologetic about their inability to define the concept (e.g., Fumerton 1995: 76–7). They will quite plausibly argue that analysis requires conceptual “atoms”—simple ideas out of which other ideas are built. The concept of acquaintance, for them, is arguably the most fundamental concept on which all epistemology, and perhaps all philosophy of mind, is built. Still, one philosopher’s conceptual atom is another’s complete mystery, and unless one can convince oneself that one understands what acquaintance is, one will not be able to understand views that invoke it.