Yehoshua - Perek Alef Sources on חֲזַ֣ק וֶֽאֱמָץ֒

Plato: The Republic, Chapter 2

• Socrates is having a conversation with Glaucon about building an ideal society. Eventually they get to the topic of creating a military/gaurdian class. Creating this class within an ideal society then takes up most of the rest of the book.
  

"Well, and your guardian must be brave if he is to fight well?
Certainly.
And is he likely to be brave who has no spirit, whether horse or dog or any other animal? Have you never observed how invincible and unconquerable is spirit and how the presence of it makes the soul of any creature to be absolutely fearless and indomitable?
I have.
Then now we have a clear notion of the bodily qualities which are required in the guardian.
True.
And also of the mental ones; his soul is to be full of spirit?
Yes.
But are not these spirited natures apt to be savage with one another, and with everybody else?
A difficulty by no means easy to overcome, he replied.
Whereas, I said, they ought to be dangerous to their enemies, and gentle to their friends; if not, they will destroy themselves without waiting for their enemies to destroy them.
True, he said.
What is to be done then? I said; how shall we find a gentle nature which has also a great spirit, for the one is the contradiction of the other?
True. He will not be a good guardian who is wanting in either of these two qualities; and yet the combination of them appears to be impossible; and hence we must infer that to be a good guardian is impossible."

http://classics.mit.edu/Plato/republic.3.ii.html