“One must do as others do”: a suspect maxim, which nearly always means: “one must do wrong” as soon as it is applied to anything beyond those purely external matters which are of no consequence, but depend on custom, fashion or convention.

[«Il faut faire comme les autres»: maxime suspecte, qui signifie presque toujours: «il faut mal faire» dès qu’on l’étend au delà de ces choses purement extérieures, qui n’ont point de suite, qui dépendent de l’usage, de la mode ou des bienséances.]

Jean de La Bruyère (1645-1696) French essayist, moralist

The Characters [Les Caractères], ch. 12 “Of Opinions [Des Jugements],” § 10 (12.10) (1688) [tr. Stewart (1970)]
    (Source)

(Source (French)). Alternate translations:

We must do like other men; a dangerous Maxim, and for the most part signifies we must do ill; if you speak not of things purely exteriour, and of no consequence, but what depends on Custome, Fashion, or Decency.
[Bullord ed. (1696)]

We must do as others do; a dangerous Maxim, which for the most part signifies we must do ill, if extended beyond things purely exterior, and of no consequence, things depending on Custom, Fashion, or Decency.
[Curll ed. (1713)]

We must do like other Men: a dangerous Maxim, for the most Part signifying we must do ill, if you carry it beyond things external, and of no consequence, but depending on Custom, Fashion, or Ceremony.
[Browne ed. (1752)]

“We must do as others do” is a dangerous maxim, which nearly always means “we must do wrong” if it is applied to any but external things of no consequence, and depending on custom, fashion, or decency.
[tr. Van Laun (1885)]