We may define all the essentials of politeness, but we cannot determine how and where they should be used; they depend on ordinary habits and customs, are connected with times and places, and are not the same in both sexes nor in different ranks of life; intelligence alone cannot find this out; politeness is acquired and perfected by imitation.
 
[L’on peut définir l’esprit de politesse, l’on ne peut en fixer la pratique: elle suit l’usage et les coutumes reçues; elle est attachée aux temps, aux lieux, aux personnes, et n’est point la même dans les deux sexes, ni dans les différentes conditions; l’esprit tout seul ne la fait pas deviner: il fait qu’on la suit par imitation, et que l’on s’y perfectionne.]

Jean de La Bruyere
Jean de La Bruyère (1645-1696) French essayist, moralist
The Characters [Les Caractères], ch. 5 “Of Society and Conversation [De la Société et de la Conversation],” § 32 (5.32) (1688) [tr. Van Laun (1885)]
    (Source)

(Source (French)). Alternate translations:

We may define Politeness, tho we can't tell where to fix it in practice. It observes received Uses and Customs, 'tis bound to times and places, and is not the same thing in the two Sexes, or in different conditions. Wit alone cannot attain it: 'tis acquired and compleated by Imitation.
[Bullord ed. (1696)]

We may define Politeness, tho we can't tell where to fix it in Practice. It observes receiv'd Uses and Customs, is bound to Times and Places, and is nto the same thing in the two Sexes, or in different Conditions; Wit alone cannot attain it, 'tis acquir'd and brought to perfection by imitation.
[Curll ed. (1713)]

It is possible to define the spirit of politeness, but not to lay down rules for its practice: it depends on custom and convention; it is related to periods and places and people, and it is not the same for the two sexes nor for various social conditions; one cannot attain it through intelligence alone, yet intelligence can enable one to imitate it, and to acquire perfection in it.
[tr. Stewart (1970)]