Etymology
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reason (v.)

c. 1400, resounen, "to question (someone)," also "to challenge," from Old French resoner, raisoner "speak, discuss; argue; address; speak to," from Late Latin rationare "to discourse," from Latin ratio "reckoning, understanding, motive, cause," from ratus, past participle of reri "to reckon, think" (from PIE root *re- "to reason, count").

The intransitive sense of "to think in a logical manner, exercise the faculty of reason" is from 1590s; transitive sense of "employ reasoning (with someone)" is from 1680s. Related: Reasoned; reasoning.

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suspicion (n.)
c. 1300, "act of suspecting; unverified conjecture of wrongdoing; mistrust, distrust," from Anglo-French suspecioun, corresponding to Old French suspicion, sospeçon "mistrust, suspicion" (Modern French soupçon), from Late Latin suspectionem (nominative suspectio) "mistrust, suspicion, fear, awe," noun of state from past participle stem of Latin suspicere "look up at" (see suspect (adj.)). Spelling in English influenced 14c. by learned Old French forms closer to Latin suspicionem. Used as a verb meaning "to suspect," it figures in literary representations of U.S. Western (Kentucky) slang from 1830s.

"Suspicion" words in other Indo-European languages also tend to be words for "think" or "look" with prefixes meaning "under, behind;" such as Greek hypopsia (hypo "under," opsis "sight"), hyponoia (noein "to think"); Lettish aizduomas (aiz "behind," duomat "think"); Russian podozrenie (Slavic podu "under," Old Church Slavonic zireti "see, look"); Dutch achterdocht (achter "behind," denken "to think").
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opine (v.)

mid-15c., opinen, "express an opinion or opinions; to think, suppose," also transitive, "be of the opinion that," from Old French oppiner, opiner (15c.) and directly from Latin opinari "have an opinion, be of opinion, suppose, conjecture, think, judge," which is of unknown origin. It is traditionally considered to be related to optare "to desire, choose" (see option), but de Vaan's sources find the evidence of this weak. Related: Opined; opining.

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deign (v.)

c. 1300, deinen, "think worthy, think well of, regard as suited to one's dignity," from Old French deignier (Modern French daigner) and directly from Latin dignari "to deem worthy or fit" (source of Italian degnare, Spanish deñar), from dignus "worthy," from PIE root *dek- "to take, accept." Sense of "take or accept graciously" led to that of "condescend" (1580s), with an infinitive for an object. Related: Deigned; deigning.

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mens rea 

"state of mind accompanying an act which condemns the perpetrator to criminal punishment," Latin, literally "guilty mind;" from mens "mind," from PIE root *men- (1) "to think."

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mentation (n.)

"mental function, the action or exercise of the mind" 1839, from Latin ment-, stem of mens "mind" (from PIE root *men- (1) "to think") + -ation. Also "a state of mind."

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incogitable (adj.)
"unthinkable, inconceivable," 1520s, from Late Latin incogitabilis "unthinking; unthinkable," from in- "not" (see in- (1)) + cogitabilis "thinkable, conceivable," from stem of cogitare "to think" (see cogitation).
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philosophize (v.)

"to think and reason about the subjects of philosophy; to meditate upon the fundamental principles of being or knowledge," 1590s, from philosophy + -ize. Related: Philosophized; philosophizing. The earlier verb was simply philosophy (late 14c., philosophien).

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amentia (n.)
"mental deficiency," late 14c., from Latin amentia "madness," from amentem "mad," from a for ab "away from" (see a- (2)) + mentem "mind" (from PIE root *men- (1) "to think") + abstract noun ending -ia.
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Clytaemnestra 
also Clytemnestra, wife and murderess of Agamemnon, from Greek Klytaimnestra, from klytos "celebrated, heard of" (see loud) + mnester "wooer, suitor," literally "willing to mind, mindful of," related to mnasthai "to remember," from PIE root *men- (1) "to think."
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