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.
Santayana wishes, according to Will Durant, to "devise a means whereby men may be persuaded to virtue without the stimulus of supernatural hopes and fears."
Families and children are immensely important too, as, "we commit the blotted manuscript of our lives more willingly to the flames, when we find the immortal text half engrossed in a fairer copy."
Leisure is critically important to a society, and necessary for the development of culture and arts, since "civilization has hitherto consisted in the diffusion and dilution of habits arising in privileged centers."
Durant says he: "achieves his masterpiece in 'Reason in Religion', filling his skeptical pages with a tender sadness, and finding in the beauty of Catholicism plentiful cause for loving it still."
Santayana is a materialist and a naturalist, and strongly dislikes the more mystical metaphysics of many of his contemporaries; he rejects even the pantheism of Spinoza, saying that "the word nature is poetical enough; it suggest sufficiently the generative and controlling function, the endless vitality and changeful order of the world in which I live."
The eternal has absorbed him while he lived, and when he is dead his influence brings others to the same absorption, making them, through that ideal identity with the best in him, reincarnations and perennial seats of all in him which he could rationally hope to rescue from destruction.
By becoming the spectator and confessor of his own death and of universal mutation, he will have identified himself with what is spiritual in all spirits and masterful in all apprehension; and so conceiving himself, he may truly feel and know that he is eternal.—Reason in Religion, p. 273.