perspicacity \pur-spuh-KAS-uh-tee\ (noun) - Clearness of understanding or insight; penetration, discernment.
"I understand he was a great help to Chex Centaur. There is also no problem about feeding him. I commend you for your perspicacity in selecting him." -- Piers Anthony, 'Heaven Cent'
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Perspicacity comes from Latin perspicax, perspicac-, "sharp-sighted," from perspicere, "to look through," from per, "through" + specere, "to look."