Do not carry Fools on your Back. He that does not know a fool when he sees him is one himself: still more he that knows him but will not keep clear of him. They are dangerous company and ruinous confidants. Even though their own caution and others’ care keeps them in bounds for a time, still at length they are sure to do or to say some foolishness which is all the greater for being kept so long in stock. They cannot help another’s credit who have none of their own. They are most unlucky, which is the Nemesis of fools, and they have to pay for one thing or the other. There is only one thing which is not so bad about them, and this is that though they can be of no use to the wise, they can be of much use to them as signposts or as warnings

Gracian, The Art of Worldly Wisdom