McCoy: A negative attitude is no good to us. We can’t just leave them out there, wherever they are. 

Spock: We shall continue sensor scans, Doctor. At the moment, that is all we can do, except hope for a rational explanation. 

McCoy: Hope? I always thought that was a human failing, Mister Spock. 

Spock: True, Doctor. Constant exposure does result in a certain degree of contamination.

Star Trek — Season 2: Episode 17 — “The Gamesters of Triskelion”