The description of the knowledge hierarchy to be presented here closely parallels that offered by Ayn Rand in her Introduction to Objectivist Epistemology (New York: New American Library, 1979), which is highly recommended to the serious student (more information). Sensation and perception—and in particular the issues of optical and other sensory "illusions"—are explored in depth by David Kelley in The Evidence of the Senses: A Realist Theory of Perception (Baton Rouge, LA: Louisiana State University Press, 1986) (more information).