Make use of your Enemies. You should learn to seize things not by the blade, which cuts, but by the handle, which saves you from harm: especially is this the rule with the doings of your enemies. A wise man gets more use from his enemies than a fool from his friends. Their ill-will often levels mountains of difficulties which one would otherwise not face. Many have had their greatness made for them by their enemies. Flattery is more dangerous than hatred, because it covers the stains which the other causes to be wiped out. The wise will turn ill-will into a mirror more faithful than that of kindness. and remove or improve the faults referred to. Caution thrives well when rivalry and ill-will are next-door neighbors.

Gracian, The Art of Worldly Wisdom