"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 try" (see conation).
Most writers make ratiocination synonymous with reasoning. J.S. Mill and others hold that the word is usually limited to necessary reasoning. [Century Dictionary]