- Popkin, R. H. The History of Skepticism from Erasmus to Descartes (rev. ed. 1968); C. L. Stough, Greek Skepticism (1969); M. Burnyeat, ed., The Skeptical Tradition (1983); B. Stroud, The Significance of Philosophical Skepticism (1984). Encyclopedia2.thefreedictionary.com.
- "Philosophical views are typically classed as skeptical when they involve advancing some degree of doubt regarding claims that are elsewhere taken for granted." utm.edu