perspicacious, perspicuous
Fowler (1926) snootily urged the use of simpler alternatives by ‘those who are neither learned nor pretentious’. Perspicacious means ‘having mental penetration or discernment, discerning’, and its corresponding noun is perspicacity. Perspicuous, on the other hand, means ‘clear to understand’ (with reference to people and statements), and its noun is perspicuity. It is the nouns that are confused rather than the adjectives, and the following examples of correct uses may help to distinguish them in the reader's mind:

• He went through the photographs. But it didn't take much perspicacity to tell that some…were missing —Ruth Rendell, 1983

• He [William Cobbett] wrote with perspicuity and vigour, in a prose style commended by Hazlitt as ‘plain, broad, downright English’ —Margaret Drabble, 2000.

Suitable alternative words are (for perspicacity) perception, perceptiveness, acuteness, and shrewdness, and (for perspicuity) clarity, lucidity, and lucidness.

Modern English usage. 2014.