Santayana builds on his Skepticism and Animal Faith, which he described as a sort of precursor to "a new system of philosophy", that would be developed fully in the present work.
Staying true to his materialism, Santayana holds matter as the "primordial existential flux" and believes it can be, at least in some sense, known.
Humans can know matter only from a distance, symbolically: Matter is in fact referred to by Santayana as a “metaphor” only, producing one of the more provocative aspects of his philosophy: science is no less literary than poetry in representing matter in that it must express its truths at a remove, through the lens of human bias.Thus, while Santayana reveres and deeply respects science (and believes it useful for everyday experiences), he does not deify it in the way many other philosophers of the 20th century have, and he limits it to a fallible approximation of truth.
Spirit is, as used by Santayana, very much akin to consciousness; it is, according to John Lachs, "part of a life constituted by its series of intuitions."
Herein is to be found the Realm of Spirit; it is Santayana's attempt to reconcile the theories of Plato and the demands of reason.