“You never know, he thought. You just never know. You drift along, year after year, presuming certain values to be fixed; like being able to drive on a public thoroughfare without somebody trying to murder you.”

"Duel" (1971), a short story, which he later adapted into a screenplay for Duel (1971), Steven Spielberg's first feature-length film.
Context: You never know, he thought. You just never know. You drift along, year after year, presuming certain values to be fixed; like being able to drive on a public thoroughfare without somebody trying to murder you. You came to depend on that sort of thing. Then something occurs and all bets are off. One shocking incident and all the years of logic and acceptance are displaced and, suddenly, the jungle is in front of you again. Man, part animal, part angel. Where had he come across that phrase? He shivered.
It was entirely an animal in that truck out there.