This chapter discusses some themes to which Lewis returned often because they reflect philosophical errors that are still influential in culture—science and scientism, evolution and evolutionism. Under the facade of science, even the science of evolution, philosophical naturalism, materialism, and reductionism serve as the paragons of knowledge and often guide social policy. Thus, “scientism” and “evolutionism” are labels for the combination of naturalism and science in general and evolutionary science in particular. Lewis defines science as seeking natural causes for natural effects, which, when successful, formulates laws of the physical operation of nature. Such an intellectual enterprise is neutral with respect to religious and theological positions and is hardly strong evidence for naturalism and empiricism. Lewis identifies the conflict as occurring, not between science and religion (or theism), but between naturalism and theism as philosophical worldviews. As a case in point, Lewis sees no conflict between the scientific theory of evolution and its increasing confirmation by empirical evidence, but he does see a conflict between evolution as interpreted by philosophical naturalism—with ideas that humanity is not of special worth, that there is no God who is ultimately responsible for the existence of the world, and so on. An item of particular interest is the Lewis–Van Osdall correspondence (recently discovered, never before published) regarding what advice Lewis would offer on Van Osdall’s contemplated book aimed at presenting science to a general audience, especially a Christian audience.