 To summarize: Russell’s distinction between knowledge by acquaintance and knowledge by description is arguably best understood as a distinction between foundational knowledge of truths acquired by acquaintance and nonfoundational knowledge of truths that depend, ultimately, on foundational knowledge. Acquaintance is a direct, nonjudgmental and nonconceptual form of awareness that he took to be crucial for both forms of knowledge. Following Russell, some appeal to acquaintance with particular facts to account for the possibility of foundational empirical knowledge; some appeal to acquaintance with universals to account for the possibility of thought itself; some appeal to acquaintance with complex universals to account of the possibility of foundational a priori knowledge; and some appeal to acquaintance with logical and probabilistic relations to account for the possibility of nonfoundational knowledge or “knowledge by description.” Thus, for Russell and many other acquaintance theorists, even “knowledge by description” depends on acquaintance, and in more than one way. Recently, more sophisticated forms of the acquaintance theory have been developed, and responses to central objections offered. Moreover, some philosophers of mind have appealed to acquaintance to either defend, or critique, the knowledge argument for dualism. In light of all this, one can expect the relation of acquaintance and the distinction between knowledge by acquaintance and knowledge by description to remain at the center of a number of debates in epistemology and the philosophy of mind. 