Few men are sufficiently discerning to appreciate all the evil they do.

[Il n’y a guère d’homme assez habile pour connoître tout le mal qu’il fait.]

François VI, duc de La Rochefoucauld (1613-1680) French epigrammatist, memoirist, noble

Réflexions ou sentences et maximes morales [Reflections; or Sentences and Moral Maxims], ¶269 (1665-1678) [tr. Tancock (1959), ¶269]
    (Source)

First appeared in the 2nd (1666) edition. In manuscript, it reads "... assez pénétrant pour apercevoir tout le mal qu’il fait."

(Source (French)). Other translations:

There are but few Men Wise enough to know all the Mischief Wisdom does.
[tr. Stanhope (1694), ¶270]

There are but few Men wise enough to know all the Mischief they do.
[tr. Stanhope (1706), Powell ed., ¶269]

Few men are able to know all the ill they do.
[pub. Donaldson (1783), ¶5]

Few men are able to know all the ill they do.
[ed. Lepoittevin-Lacroix (1797), ¶252]

Few of us have abilities to know all the ill we occasion.
[ed. Carvill (1835), ¶3]

Scarcely any man is clever enough to know all the evil he does.
[ed. Gowens (1851), ¶280]

No man is clever enough to know all the evil he does.
[tr. Bund/Friswell (1871), ¶269]

No one is sufficiently keen to realize to the full the harm he does.
[tr. Heard (1917), ¶277]

Scarcely any man is clever enough to realize all the harm he does.
[tr. Stevens (1939), ¶269]

There is hardly a man clever enough to recognize the full extent of the evil that he does.
[tr. FitzGibbon (1957), ¶269]

Almost no one is perceptive enough to realize all the harm he does.
[tr. Kronenberger (1959), ¶269]

There is scarcely a man alive clever enough to know all the evil he does.
[tr. Whichello (2016) ¶269]