Character Analysis
No Holds Barred
If Spock is Kirk's superego, all cold logic and impassive observation, Dr. Leonard "Bones" McCoy is his raging id.
The ship's doctor has never been shy about expressing his opinion…or about letting others know when they're falling down on the job. That gives him the ability to say things to both Kirk and Spock that no one else can say: to call them on their horse-pucky and make sure they know what they've been hiding from.
He gives it to Kirk first, while Kirk is wallowing in self-pity on his birthday.
McCOY: What the hell do you want? This is not about age, and you know it. This is about you flying a goddamn computer console when you wanna be out there hopping galaxies.
KIRK: Spare me your notions of poetry, please. We all have our assigned duties.
McCOY: Bull. You're hiding: hiding behind rules and regulations.
KIRK: Who am I hiding from?
McCOY: From yourself, Admiral!
Later in the film, Spock gets the same treatment, when McCoy suspects him of dodging the moral implications of the Genesis Device.
McCOY: But, dear Lord, do you think we're intelligent enough to... suppose… what if this thing were used where life already exists?
SPOCK: It would destroy such life in favor of its new matrix.
McCOY: It's new matrix? Do you have you any idea what you're saying?
In literary terms, that makes Bones a foil—a character who challenges and tests the hero by forcing him to look at things in a particular way.
It's also, we might add, one of the great things about the character in the original TV show…and something that the first Star Trek movie completely glossed over. Fans were quite relieved so see McCoy's old grumpy self come back, especially because he wasn't going to let Kirk or Spock get away with anything.