Perspicacity (also called perspicaciousness and perspicuity) is a penetrating discernment—a clarity of vision or intellect which provides a deep understanding and insight.
In the 17th century, René Descartes devised systematic rules for clear thinking in his work Regulæ ad directionem ingenii (Rules for the direction of natural intelligence). In Descartes' scheme, intelligence consisted of two faculties: perspicacity, which provided an understanding or intuition of distinct detail; and sagacity, which enabled reasoning about the details in order to make deductions. Rule 9 was De Perspicacitate...