1620s, "one who follows reason and not authority in thought or speculation," especially "physician whose treatment is based on reasoning," from rational + -ist. In theology/philosophy, "one who applies rational criticism to the claims of supernatural authority or revelation," 1640s. This sense shades into that of "one who believes that human reason, properly employed, renders religion superfluous." Related: Rationalistic; rationalism (1800 in medicine; 1827 in theology, "adherence to the supremacy of reason in matters of belief or conduct;" by 1876 in general use).
updated on April 29, 2021