late 14c., resouning, "exercise of the power of reason; act or process of thinking logically;" also an instance of this, a presentation of reasons or arguments; verbal noun from reason (v.).
"process of reasoning, mental process of passing from the cognition of premises to the cognition of the conclusion," 1520s, from Latin ratiocinationem (nominative ratiocinatio) "a reasoning, calm reasoning," noun of action from past-participle stem of ratiocinari "to reckon, compute, calculate; to deliberate, meditate; to reason, argue, infer." This is a compound of ratio "reckoning, calculation," also "judgment, reason" (see ratio) + -cinari, which probably is related to conari "to endeavor, to try," from PIE *kona-, from root *ken- "to hasten, set oneself in motion" (see deacon).
Most writers make ratiocination synonymous with reasoning. J.S. Mill and others hold that the word is usually limited to necessary reasoning. [Century Dictionary]
mid-14c., logike, "branch of philosophy that treats of forms of thinking; the science of distinction of true from false reasoning," from Old French logique (13c.), from Latin (ars) logica "logic," from Greek (he) logike (techne) "(the) reasoning (art)," from fem. of logikos "pertaining to speaking or reasoning" (also "of or pertaining to speech"), from logos "reason, idea, word" (see Logos). Formerly also logick. Sometimes formerly plural, as in ethics, but this is not usual. Meaning "logical argumentation" is from c. 1600. Contemptuous logic-chopper "sophist, person who uses subtle distinctions in argument" is from 1846.
1640s, "relating to the art of reasoning about probabilities," from Latin dialecticus, from Greek dialektikos "of conversation, discourse," from dialektos "discourse, conversation" (see dialect). From 1813 as "of or pertaining to a dialect or dialects."
late 14c., "process of understanding, reasoning, thought," from French discours, from Latin discursus "a running about," in Late Latin "conversation," in Medieval Latin "reasoning," noun use of past participle of discurrere "to run about, run to and fro, hasten," in Late Latin "to go over a subject, speak at length of, discourse of," from dis- "apart" (see dis-) + currere "to run" (from PIE root *kers- "to run").
Meaning "a running over a subject in speech, communication of thought in words" is from 1550s; sense of "discussion or treatment of a subject in formal speech or writing," is from 1580s.