Zarabeth: But your friend--he is ill.
Spock: That is true. If I leave him, he may never regain the ship. He would be marooned in this time period. But he is no longer in danger of death, so my primary duty to him has been discharged. And if I remain here, no one of our party would be able to aid Captain Kirk.
Zarabeth: Oh, you make it sound like an equation.
Spock: It should be an equation! I should be able to resolve this problem logically.