"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 neighbours." - Balthasar Gracian's "The Art of Worldly Wisdom"
Friends are useful but not to be used. They ought not to be overly taxed for their friendship, nor inconsiderately enlisted in each other's causes. Their help is given, not taken. However, whereas in friendship men consort for their mutual good, as enemies they consent to receive each other's ill-will to be manipulated and abused. What is more, enemies are more reliable. For enemies can be trusted to be untrustworthy, and their rivalry, by exploiting each other's faults, forces self-reflection upon faults. You need an enemy more than a friend. But choose your enemies wisely, those who are useful and you are capable of using.