ra ‧ tion ‧ al ‧ ist /ˈræʃ ə nələst, ˈræʃ ə nəlɪst/ BrE AmE noun [countable]
[ Word Family: noun : ↑ rationality ≠ ↑ irrationality , ↑ rationale , ↑ rationalism , ↑ rationalist , ↑ rationalization ; adjective : ↑ rational ≠ ↑ irrational , ↑ rationalist ; verb : ↑ rationalize ; adverb : ↑ rationally ≠ ↑ irrationally ]
someone who bases their actions on rationalism
—rationalist , rationalistic /ˌræʃ ə nəˈlɪstɪk◂/ adjective