Errol Morris writes: Josiah “Tink” Thompson, the subject of this Op-Doc, graduated from Yale in 1957, became a demolitions expert and frogman for the Navy, and then returned to Yale to get his Ph.D. on the Danish philosopher Soren Kierkegaard.

The Kennedy assassination changed Tink’s life. In 1967, he was an assistant professor of philosophy at Haverford, when he published “Six Seconds in Dallas.” Short, simple and quietly convincing, it is still one of the best books written about the assassination.

Ten years later, Tink left academia and became a private detective in Northern California. Now he has returned to what has haunted him for 50 years: Frame #313 of the Zapruder film, and our inability to come up with a definitive account of what happened in Dallas.