Knowledge by Acquaintance and Knowledge by Description

Acquaintance requires no intermediary: to be acquainted with something is to be in direct cognitive contact with it, with no description or inference standing between knower and known. Russell holds that we are acquainted with our own present sense-data (the patch of colour I see), with our own mental states (this present desire or memory), and — controversially — with universals (the relation of resemblance itself, grasped directly when I notice that two patches of colour resemble one another). Physical objects, other minds, and most things in the past and future are not available to acquaintance; they are known only through descriptions.

To know something by description is to know it as "the so-and-so" — as the thing that uniquely satisfies some condition. I know Julius Caesar as the Roman general who crossed the Rubicon and was stabbed in the Senate; I know electrons as the smallest negatively charged particles. This knowledge is perfectly genuine, but it is indirect: I know about Caesar rather than knowing Caesar himself. Russell argues that all knowledge that extends beyond our immediate experience is ultimately knowledge by description, resting on foundations of acquaintance that give the descriptions their meaning.

The distinction has a deeper significance: it identifies what language ultimately refers to. Every meaningful word must, following Russell's principle of acquaintance, be either a name for something we are directly acquainted with, or be analysable into such names. This principle — one of the foundations of analytic philosophy — implies that the terms of our language trace back, through chains of definition and description, to the sense-data and universals that are the bedrock of direct experience. Without this bedrock, language would be a system of symbols pointing only to other symbols, with no ultimate grip on reality.

The distinction between knowledge by acquaintance and knowledge by description first appears in "Knowledge by Acquaintance and Knowledge by Description" (1910) and is developed in The Problems of Philosophy, Chapter 5. It became one of the most discussed distinctions in twentieth-century epistemology.