Etymology
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skeptic (n.)

also sceptic, 1580s, "member of an ancient Greek school that doubted the possibility of real knowledge," from French sceptique and directly from Latin scepticus "the sect of the Skeptics," from Greek skeptikos (plural Skeptikoi "the Skeptics, followers of Pyrrho"), noun use of adjective meaning "inquiring, reflective" (the name taken by the disciples of the Greek philosopher Pyrrho, who lived c. 360-c. 270 B.C.E.), related to skeptesthai "to reflect, look, view" (from PIE root *spek- "to observe").

Skeptic does not mean him who doubts, but him who investigates or researches as opposed to him who asserts and thinks that he has found. [Miguel de Unamuno, "Essays and Soliloquies," 1924]
There is one word of caution, however, to be given to those who renounce inquiry; it is that they cannot retain the right to condemn inquirers. [Benjamin Jowett, "On the Interpretation of Scripture," in "Essays and Reviews," 1860]

The extended sense of "one with a doubting attitude" first recorded 1610s. The sk- spelling is an early 17c. Greek revival and is preferred in U.S. As a verb, scepticize (1690s) failed to catch on.

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skepticism (n.)
also scepticism, 1640s, from skeptic + -ism. Specifically regarding Christian religion, from 1800.
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sceptic (n.)
chiefly British English spelling of skeptic (q.v.). Related: Sceptical; sceptically; scepticism.
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skeptical (adj.)
also sceptical, 1630s; see skeptic + -al (1). Related: Skeptically.
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*spek- 
Proto-Indo-European root meaning "to observe."

It forms all or part of: aspect; auspex; auspices; auspicious; bishop; circumspect; conspicuous; despicable; despise; episcopal; especial; espionage; espy; expect; frontispiece; gyroscope; haruspex; horoscope; inspect; inspection; inspector; introspect; introspection; perspective; perspicacious; perspicacity; prospect; prospective; respect; respite; retrospect; scope; -scope; scopophilia; -scopy; skeptic; species; specimen; specious; spectacle; spectacular; spectrum; speculate; speculation; speculum; spice; spy; suspect; suspicion; suspicious; telescope.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit spasati "sees;" Avestan spasyeiti "spies;" Greek skopein "behold, look, consider," skeptesthai "to look at," skopos "watcher, one who watches;" Latin specere "to look at;" Old High German spehhon "to spy," German spähen "to spy."
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philosophist (n.)

"a would-be philosopher," a disparaging term for a rationalist or skeptic, a philosophe; 1798, from French philosophiste; see philosophy + -ist.

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philosophe (n.)

also philosoph, "Enlightenment rationalist and skeptic," especially in reference to any of the French Encyclopædists, often disparaging or with contemptuous implication (when used by believers), 1774, from French philosophe, literally "philosopher" (Old French filosofe; see philosopher). Usually italicized in English, but nativized by Peter Gay ("The Enlightenment," 1966) and others. Also compare philosophist. It also was the older word for "philosopher" in English, from Old English to c. 1400.

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Pyrrhonic (adj.)

1590s, "pertaining to Pyrrho" (Greek Pyrrhōn, c. 360-c. 275 B.C.E.), skeptic philosopher of Elis, who held the impossibility of attaining certainty of knowledge. The name means "reddish" or "red-haired," from pyrrhos "flame-colored," from pyr "fire" (from PIE root *paewr- "fire"). Related: Pyrrhonism; Pyrrhonist.

The doctrine of Pyrrho was that there is just as much to be said for as against any opinion whatever ; that neither the senses nor the reason are to be trusted in the least ; and that when we are once convinced we can know nothing, we cease to care, and in this way alone can attain happiness. It is said that Pyrrho would take no ordinary practical precautions, such as getting out of the way of vehicles. [Century Dictionary]
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