rationalist

noun

1. A person who follows the philosophy of rationalism.

   ““He says,” replied Lord Peter, “that he believes Hannah to be a sincerely religious woman. He has sat beside her in chapel and shared her hymn-book.” “But that may be the merest hypocrisy,” said Miss Murchison, rather warmly, for she was militantly rationalist. “I don’t trust these unctuous people.””

2. A member of the rationalist community.

   “Some rationalists were in touch with Oxford philosophers, including Toby Ord and William MacAskill, the founders of the effective-altruism movement, which studied how to do the most good for humanity (and, by extension, how to avoid ending it).”