Mr. Spock : You're the captain of this ship. You haven't the right to be vulnerable in the eyes of the crew. You can't afford the luxury of being anything less than perfect. If you do, they lose faith and you lose command.
Spock : If I seem insensitive to what you're going through, Captain, understand - it's the way I am.
Lt. Cmdr. Leonard 'Bones' McCoy, M.D. : Jim, you can't risk your life on a theory!
Mr. Spock : Being split in two halves is no theory with me, Doctor. I have a human half, you see, as well as an alien half, submerged, constantly at war with each other. Personal experience, Doctor. I survive it because my intelligence wins out over both, makes them live together.
Mr. Spock : Your intelligence would enable you to survive, as well.
Captain James T. Kirk : What's the matter with me?
Mr. Spock : Judging from my observations, Captain, you're rapidly losing the power of decision.
Lt. Cmdr. Leonard 'Bones' McCoy, M.D. : You have a point, Spock?
Mr. Spock : Yes. Always, Doctor. We have here an unusual opportunity to appraise the human mind, or to examine, in Earth terms, the roles of good and evil in a man: his negative side, which you call hostility, lust, violence, and his positive side, which Earth people express as compassion, love, tenderness.
Lt. Cmdr. Leonard 'Bones' McCoy, M.D. : It's the captain's guts you're analyzing. Are you aware of that, Spock?
Mr. Spock : Yes, and what is it that makes one man an exceptional leader? We see indications that it is his negative side which makes him strong, that his "evil" side, if you will, properly controlled and disciplined, is vital to his strength.
Mr. Spock : Your negative side removed from you, the power of command begins to elude you.
Captain James T. Kirk : What is your point, Mr. Spock?
Mr. Spock : If your power of command continues to weaken, you'll soon be unable to function as captain. You must be prepared for that.