It’s April 14, 1961, Washington D.C., at the Potomac Club. A group of well-heeled club members play cards and discuss the metaphysics of time travel. One argues that if time travel were possible, one could prevent a financial disaster that we know occurred in the past, from taking place. Another disagrees: If an event has taken place in the past, then it can’t be altered or be prevented from happening. It is this second claim, that the episode sets out to defend, with a qualification.
Mr.Peter Corrigan is a participant in this debate, and when he leaves the table, he inexplicably finds that he has time-traveled to April 14, 1865, hours before Abraham Lincoln’s assassination at Ford’s Theatre. He attempts to prevent the assassination – wouldnt’ you? – but he fails, as he must. Yet on his reappearance in the present, we discover that something that is the case, the great wealth of one of his fellow club members, is causally linked to his attempt to prevent Lincoln’s assassination. The conclusion: “In the matter of time travel, gentlemen, some things can be changed; some things can’t.”
So what can be changed and what can’t? The short answer is that you cannot bring it about that not-p when it is true that p, and you can bring it that p when it is true that p. When Corrigan travels back in time it is true both that Lincoln was assassinated in 1865, and that his club colleague is wealthy. Prevented from changing the past only precludes him from changing those things that will alter the truths about the past.
Of course, the short answer raises questions that don’t have short answers. How can Corrigan be a contributing cause in the acquisition of his colleague’s long-deceased ancestor’s wealth? Corrigan (the cause) comes into existence after the accumulation of wealth (the effect). So we have to tolerate causes occurring after their effects.
If we must adhere to the requirement that the past can’t be altered, must we also adhere to the requirement that causes must occur prior to their effects? “Back There” doesn’t explicitly address this, but it suggests that there is a difference between these two principles. Violating the first is unthinkable. Violating the second is thinkable, when we imagine the story of Corrigan and his interactions with his 19th century and his 20 century compatriots.