doctrinaire

Short definition from Wiktionary

Noun

1. 1
   A person who stubbornly holds to a philosophy or opinion regardless of its feasibility.
2. 2
   In France, in 1815–30, one of a school who desired a constitution like that of Britain.

Adjective

1. 1
   Stubbornly holding on to an idea without concern for practicalities or reality.