 In addition to the above titles by Russell, Russell’s most influential writings relating to his theories of metaphysics and epistemology include Our Knowledge of the External World (1914a), “The Relation of Sense-Data to Physics” (1914c), “The Philosophy of Logical Atomism” (1918, 1919), “On Propositions: What They Are and How They Mean” (1919b) and An Outline of Philosophy (1927b).