Plutarch De capienda ex inimicis utilitate 7

[7] Whenever, then, anything untrue has been said, you must not despise and disregard it just because it is false, but rather consider what word or act of yours, which of your pursuits or associations, has given colour to the calumny, and then be studiously careful to avoid it. For if others by becoming involved in undesired situations thereby learn a useful lesson—just as Merope says that what is to hinder a man from taking his enemy as his teacher without fee, and profiting thereby, and thus learning, to some extent, the things of which he was unaware ? For there are many things which an enemy is quicker to perceive than a friend (for Love is blind regarding the loved one, as Plato ² says), and inherent in hatred, along with curiosity, is the inability to hold one’s tongue. Hiero ³ was reviled by ⁴ one of his enemies for his offensive breath; so when he went home he said to his wife, What do you mean ? Even you never told me of this. But she being virtuous and innocent said, I supposed that all men smelt so. Thus it is that things which are perceptible, material, and evident to all the world, may sooner be learned from our enemies than from our friends and close associates.