1900 Interpretations of Poetry and Religion published.

1904 Santayana sails from New York to Plymouth, England, in mid-July; visits Paris, Rome, Venice, Naples, Pompeii, Sicily, and Greece.

1905 Visits Egypt, Palestine, Tel Aviv, Jerusalem, Damascus, Baalbeck, Beirut, Athens, Constantinople, Budapest, and Vienna. While still abroad, Santayana invited by Harvard to become Hyde Lecturer at the Sorbonne for 1905–6. First four volumes of The Life of Reason; or, the Phases of Human Progress published.

1906 Fifth volume of The Life of Reason published. Santayana returns to America in September; resumes teaching at Harvard.

1907 Santayana promoted from assistant professor to full professor.