George Santayana (1937). “The Works of George Santayana: Scepticism and animal faith. Some meanings of the word "is". Literal and symbolic knowledge. The unknowable”
Durant Drake, Arthur Oncken Lovejoy, James Bissett Pratt, Arthur Kenyon Rogers, George Santayana (1920). “Essays in critical realism: a co-operative study of the problem of knowledge”
George Santayana (1937). “The Works of George Santayana: Scepticism and animal faith. Some meanings of the word "is". Literal and symbolic knowledge. The unknowable”