McCoy and Spock in “Unfinished Business”

McCoy

Jim, you can't risk your life on a theory!

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. Your intelligence would enable you to survive as well.