The exact synonym of "cognoscible" is the far better-known "cognizable." Both words mean "capable of being judicially heard and determined" (as "a cognoscible claim") and "capable of being known" (as "cognoscible circumstances"). Both terms are from Latin cognoscere, meaning "to know." And both appeared in the 17th century, less than two decades apart - first, "cognoscible," direct from the Late Latin adjective cognoscibilis; then, "cognizable," from the English noun "cognizance" ("knowledge").