As a matter of self-preservation, a man needs good friends or ardent enemies, for the former instruct him and the latter take him to task.

 

Diogenes Diogenes  ”the Cynic”, Greek philosopher, was born in Sinope (modern day Sinop, Turkey) about 412 BC (according to other sources 404 BC), and died in 323 BC, at Corinth.

We read that we ought to forgive our enemies; but we do not read that we ought to forgive our friends.

Francis Bacon

In the End, we will remember not the words of our enemies, but the silence of our friends.

Martin Luther King, Jr.(1929 –  1968)  An African American clergyman, activist and prominent leader in the American civil rights movement.

I no doubt deserved my enemies, but I don’t believe I deserved my friends.

Walt Whitman, (1819 –1892) An American poet, essayist, journalist, and humanist

I choose my friends for their good looks, my acquaintances for their good characters, and my enemies for their intellects. A man cannot be too careful in the choice of his enemies.

Oscar Wilde

He has no enemies, but is intensely disliked by his friends. 

Oscar Wilde

Men of sense often learn from their enemies. It is from their foes, not their friends, that cities learn the lesson of building high walls and ships of war. 

Aristophanes

Except in cases of necessity, which are rare, leave your friend to learn unpleasant things from his enemies; they are ready enough to tell them.

Oliver Wendell Holmes, ( 1841 –  1935) An American jurist.