Teaching an ass to obey the rein, which the ancients thought to be nearly as difficult as to wash a blackamoor white, or to do any other impossible thing, «Labour in vain». Though I think it is not now found to be so difficult, and those animals are made to serve for many useful purposes. The adage is used by Horace, and with much elegance, in his first Satire.
«At si cognatos nullo natura labore
Quos tibi dat, retinere velis, servareque amicos;
Infelix operam perdas; ut si quis asellum
In campo doceat parentem currere frœnis».
But if you expect to obtain the affection of your relations, or to preserve the esteem of your friends, without making any return for their kindness, you will find yourself, wretch that you are, miserably deceived, as he would be, who should attempt to reach an ass to be obedient to the rein.