Knowledge is defined by Locke as ‘the perception of agreement or disagreement of two ideas’.1 There are four sorts of agreement: identity (or diversity); relation; coexistence or necessary connection; and real existence.2 Of these four, the standard case of knowledge is the second. Knowledge of identity and diversity (white is white, white is not red) is ‘trifling’. Knowledge of co-existence (by which Locke means the coexistence of qualities in an object) is unattainable. Knowledge of real existence is very limited, and also dubiously described as knowledge of an agreement between two ideas. With these latter categories we shall be concerned later.
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