The Life of Reason: The Phases of Human Progress is a book published in five volumes from 1905 to 1906, by Spanish-born American philosopher George Santayana. It consists of Reason in Common Sense, Reason in Society, Reason in Religion, Reason in Art, and Reason in Science.

The work is considered to be the most complete expression of Santayana's moral philosophy; by contrast, his later magnum opus, the four-volume The Realms of Being, more fully develops his metaphysical and epistemological theory, particularly his doctrine of essences. Santayana's philosophy is strongly influenced by the materialism of Democritus and the refined ethics of Aristotle, with a special emphasis on the natural development of ideal ends.

The Life of Reason is sometimes considered to be one of the most poetic and well-written works of philosophy in Western history. To supply but a single example, the oft-quoted aphorism of Santayana's, "Those who cannot remember the past are condemned to repeat it," may be found on p. 284 of Reason in Common Sense.

In 1951, near the end of his life, Santayana engaged himself in the weighty task of producing a one-volume abridgment of The Life of Reason at the urging of his editor at Scribner's, with the assistance of his friend and student, Daniel Cory. As Cory writes in the volume's preface, in addition to excising prolixities and redundancies from the book, "[a] sustained effort was made to dispel those early mists of idealism from the realistic body of his philosophy, and to make clear to the reader that our idea of a natural world can never be that world itself."