doctrinaire

/D AA0 K T R AH0 N EH1 R/
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.

Source: https://en.wiktionary.org/wiki/doctrinaire