[adj] having the capacity for thought and reason especially to a high degree; "is there intelligent life in the universe?"; "an intelligent question"
[adj] endowed with the capacity to reason
[adj] exercising or showing good judgment; "healthy scepticism"; "a healthy fear of rattlesnakes"; "the healthy attitude of French laws"; "healthy relations between labor and management"; "an intelligent solution"; "a sound approach to the problem";"sound advice"; "no reasonable explanation for his decision"
[adj] possessing sound knowledge; "well-informed readers"
agile, alert, apt, born(p), brainy, bright, brilliant, clever, healthy, incisive, innate(p), keen, levelheaded, natural, nimble, prehensile, quick, rational, ready, reasonable, reasoning(a), scintillating, searching, sensible, sharp, smart, smart as a whi