early 15c., "in, of, or pertaining to the mind; characteristic of the intellect," from Late Latin mentalis "of the mind," from Latin mens (genitive mentis) "mind," from PIE root *men- (1) "to think."
In Middle English, also "of the soul, spiritual." From 1520s as "done or performed in the mind." Meaning "crazy, deranged" is by 1927, probably from combinations such as mental patient (1859); mental hospital (1891). Mental health is attested by 1803; mental illness by 1819; mental retardation by 1904.
"to go in procession," 1814, "A colloquial or humorous back-formation" from procession [OED]. Accent on second syllable. The earlier verb was procession (1540s).
early 14c., proces, "fact of being carried on" (as in in process), from Old French proces "a journey; continuation, development; legal trial" (13c.) and directly from Latin processus "a going forward, advance, progress," from past-participle stem of procedere "go forward" (see proceed).
Meaning "course or method of action, continuous action or series of actions or events" is from mid-14c.; sense of "continuous and regular series of actions meant to accomplish some result" (the main modern sense) is from 1620s. Meaning "a projection from the main body of something," especially a natural appendage, is from 1570s. Legal sense of "course of action of a suit at law, the whole of the proceedings in any action at law" is attested from early 14c.; hence due process "fair treatment" at law, considered as a right (mid-15c.).
1530s, "begin legal action against, summon in a court of law," from French processer "to prosecute," from proces (see process (n.)). Meaning "prepare or treat by special process, subject to special process" is from 1881, from the noun in English. Of persons, "to register and examine," by 1935, in reference to the U.S. Army. Related: Processed; processing.
mid-15c., cognicioun, "ability to comprehend, mental act or process of knowing," from Latin cognitionem (nominative cognitio) "a getting to know, acquaintance, knowledge," noun of action from past participle stem of cognoscere "to get to know, recognize," from assimilated form of com"together" (see co-) + gnoscere "to know" (from PIE root *gno- "to know"). In 17c. the meaning was extended to include perception and sensation.
1839, "arrangement (of a building, etc.) to face east or any other specified direction," noun of action from orient (v.). Meaning "process of determining the points of the compass is by 1868, hence the extended sense of "action of determining one's mental bearings," with reference to new ideas, etc. (1870). Meaning "introduction to a situation" is from 1942. Sense of "the position or arrangement (of something) relative to the points of the compass" is from 1875. Related: Orientational.
"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]