Whenever a nation changes its customs and manners it does so either out of necessity, out of boredom or because of a shock. By customs, I mean the results of the emotions and passions which move men; by manners, I mean their external behaviour, that is to say, that language of action which every society possesses to allow men to exploit these same results and show them off to their best advantage.

Necessity alters customs more than manners; and boredom alters manners more than customs; shock affects both equally. The changes brought about by necessity are rarer but more stubborn and durable; those brought about by boredom are more frequent but less lasting; those resulting from shock may be either durable or short-lived, rare or frequent, according to whether they are inspired by deep-rooted natural feelings or artificial sentiments.

Necessity has a greater influence on customs than on manners, because the former derive from the succession of human needs and the latter from the succession of opinions; and needs command whilst opinions advise. Opinions gain their ascendancy as a result of mental laziness: we are much happier sticking with the falsehoods we know than exploring new truths, and we allow ourselves to be beguiled by the ease with which something can be imagined or done, without assessing the clarity of our imaginings or the usefulness of our deeds.