noun
positivist
A person who adheres to positivism, the philosophical doctrine that only scientific knowledge derived from empirical evidence and logical reasoning is valid.
彼は実証主義者として、形而上学的な議論を退ける。
As a positivist, he rejects metaphysical arguments.
実証主義 is the doctrine of positivism, while 実証主義者 refers to a person who follows that doctrine.
Compound of 実証 (empirical proof) + 主義 (doctrine) + 者 (person). The term is a calque from Western philosophy.