supposition (n.)

early 15c., a term in logic, "assumption, hypothesis," from Medieval Latin suppositionem (nominative suppositio) "assumption, hypothesis, a supposition," noun of action from past participle stem of supponere (see suppose); influenced by Greek hypothesis. In classical Latin, "a putting under, substitution." Earlier in English in the same sense was supposal (late 14c.). Related: Suppositional; suppositionally.