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 Intuitionis (On the Perspicacity of Intuition). He summarised the rule as
Oportet ingenii aciem ad res minimas et maxime faciles totam convertere, atque in illis diutius immorari, donec assuescamus veritatem distincte et perspicue intueri.
We should totally focus the vision of the natural intelligence on the smallest and easiest things, and we should dwell on them for a long time, so long, until we have become accustomed to intuiting the truth distinctly and perspicuously.
In his study of the elements of wisdom, the modern psychometrician Robert Sternberg identified perspicacity as one of its six components or dimensions; the other five being reasoning, sagacity, learning, judgement and the expeditious use of information. In his analysis, perspicacity was described as
...has intuition; can offer solutions that are on the side of right and truth; is able to see through things — read between the lines; has the ability to understand and interpret his or her environment.
To quote an Academy brochure, the quality most needed by a scientist-astronaut is "perspicacity." He must, the brochure says, be able to quickly pick out, from among the thousands of things he sees, those that are significant, and to synthesize observations and develop and test working hypotheses.
Being perspicacious about other people, rather than having false illusions, is a sign of good mental health. The quality is needed in psychotherapists who engage in person-to-person dialogue and counseling of the mentally ill.
The artist René Magritte illustrated the quality in his 1936 painting Perspicacity. The picture shows an artist at work who studies his subject intently: it is an egg. But the painting which he is creating is not of an egg; it is an adult bird in flight.