Perspicacious

perspicacious [pûr-spĭ-KAY-shəs]
adj. having keen mental perception and understanding; discerning; acutely insightful and wise

[From Latin perspicere ("to look through") from per- ("through") specere ("to look")+ ]

The spic in perspicacious, conspicuous and suspicious has the same root as the spect in common English words like inspect and respect. In all of them, the root means "to look."

Aspect is "a way in which something is looked at." To be circumspect is "looking around; cautious and careful not to take risks." Something conspicuous is "easy to look at; or easy to see." A conspectus is "a general survey, one which looks fully at a subject." To inspect is "to look into; to examine carefully." To introspect is "to look inside oneself; to reflect on one's own thoughts and feelings." To prospect is "to look forward; to search for something (as minerals)." To respect is "to look back at (in admiration); to feel or show deferential regard for." A suspect is "someone you look at from below."

In the New York Times's obituary of William F. Buckley, Douglas Martin credits the prolific author and founder of The National Review with perspicacity:

William F. Buckley Jr., who marshaled polysyllabic exuberance, famously arched eyebrows and a refined, perspicacious mind to elevate conservatism to the center of American political discourse, died Wednesday at his home in Stamford, Conn. He was 82.