"process of reasoning," 1520s, from Latin ratiocinationem (nominative ratiocinatio) "a reasoning, calm reasoning," from past participle stem of ratiocinare "to calculate, deliberate," from ratio (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]