Santayana retired from Harvard in 1912 and spent the remainder of his life writing and traveling in Europe. His main publications are The Sense of Beauty (1896), Interpretations of Poetry and Religion (1900), The Life of Reason (five volumes, 1905–1906), Skepticism and Animal Faith (1923), The Realms of Being (four volumes, 1927–1940), Persons and Places (1944), The Middle Span (1945), and My Host the World (1953). In addition to numerous other books and essays, Santayana’s published correspondence to over 350 respondents runs to eight volumes.

Other than the fact that Josiah Royce (1855–1916) was his teacher and C.I. Lewis (1883–1964) argued against his intuitive theory of knowledge, it is not always clear how Santayana was a pragmatist. The philosophical convention places him within that group, mostly due to time and place and the pragmatist philosophers he interacted with while living in the United States. Still, Santayana’s ideas about aesthetics, reason, philosophy itself, and human nature share a common spirit with William James (1842–1910) and John Dewey (1859–1952).

Santayana’s theory of aesthetics was that beauty is the experience of pleasure in the form of an object, rather than the effects on the sense organs of the person experiencing the artwork. He claimed that all preference is basically irrational and that values are based on pleasure. His take on reason emphasized human creativity in science, religion, society, and ordinary life, as well as more obviously in art. Overall, he identified human beings as animals, inhabiting a physical world, oriented toward food, and fearful of danger. Santayana thought that nature was a kind of backdrop within which we have our experience. In The Life of Reason (1905–1906), he described nature as “drawn like a sponge, heavy and dripping from the waters of sentience.” The “nature of nature” is thus conditioned by our experience of it. These views characterize his early work.

After he left Harvard, Santayana wrote about metaphysics and ontology, emphasizing objective reality as opposed to human experience. But Santayana himself did not acknowledge this change in his subject matter, and in his later writing he claimed to be providing a more comprehensive and rigorous foundation for his earlier theories of art and experience.