  Santayana explicates the primary distinguishable characteristics of our knowledge in his four-book Realms of Being. Believing that philosophical terminology should have historical roots, Santayana employed classical terminology for these characteristics: matter, essence, spirit, and truth. And although these terms are central to many philosophical traditions, he views his work as “a revision of the categories of common sense, faithful in spirit to orthodox human tradition, and endeavouring only to clarify those categories and disentangle the confusions that inevitably arise …” (Realms 826).