doctrinaire (n.)

"one who theorizes without sufficient regard to practical considerations; one who explains things by one narrow set of theories, disregarding all other forces at work," 1820, from French doctrinaire "impractical person," originally "adherent of doctrines" (14c.), from Latin doctrina "teaching, body of teachings, learning" (see doctrine).

At first used in the context of French politics, the French word having been contemptuously applied by rival factions to those who tried to reconcile liberty with royal authority after 1815. Hence, anyone who applies doctrine without making allowance for practical considerations (1831). As an adjective, "characteristic of an impractical theorist, insisting upon the exclusive importance of one narrow theory," from 1834. Related: Doctrinairism.