- rational
- late 14c., "endowed with reason," from L. rationalis "of or belonging to reason, reasonable," from ratio (gen. rationis) "reckoning, calculation, reason" (see ratio). Rationalist "physician whose treatment is based on reason" is from 1620s; applied to a philosophical doctrine 1640s. Rationalize is first recorded 1803, "to explain, to make reasonable;" in the psychological sense of "to give an explanation that conceals true motives" it dates from 1922.