If you woke up this morning and asked yourself, “Would conservative podcaster Ben Shapiro travel backwards through kill and murder Adolf Hitler when Hitler was a tiny baby?”, then (a) you should seek help but (b) I have an answer to your question: Shapiro would not kill Baby Hitler, as he explained to a presumably confused crowd at the anti-abortion March for Life:

Ben Shapiro at March for Life: "The truth is no pro-life person would kill baby Hitler. Baby Hitler was a baby." pic.twitter.com/PPgQcbWSrq — jordan (@JordanUhl) January 18, 2019

Shapiro’s baffling remarks on Friday revived a debate that first became a mass public concern in October 2015, when the New York Times Magazine decided to tweet this for some reason:

We asked @nytmag readers: If you could go back and kill Hitler as a baby, would you do it? (What's your response?) pic.twitter.com/daatm12NZC — NYT Magazine (@NYTmag) October 23, 2015

Given certain assumptions, this isn't a hard question. Assume that going back in time merely eliminates Hitler, and that the sole effect of that is that the Nazi Party lacks a charismatic leader and never takes power in Germany, and World War II and the Holocaust are averted, and nothing worse than World War II transpires in this alternate reality, and there are no unintended negative consequences of time travel. Then the question is reduced to, "Is it ethical to kill one person to save 40-plus million people?" That's pretty easy. You don't have to be a die-hard utilitarian to think one baby is an acceptable price to pay to save tens of millions of lives.

But, of course, those assumptions are strong. Too strong. Here are just a few of the issues you'd need to sort out before even starting to intelligently consider whether killing baby Hitler would be wise.

Can time travel actually change history?

The first question here is whether backwards time travel is actually functionally possible. This is a different question from whether it's technically possible. It seems quite plausible that backward time travel could exist but that it would be impossible to actually change the course of history using it. This is how time travel is depicted in movies like 12 Monkeys or The Terminator, where, in my colleague Matt Yglesias's words, "temporal jumping simply turns out to be a feature of a universe that is nonetheless an unchanging four-dimensional block." In Terminator, for example, Kyle Reese is sent back to protect Sarah Connor, because her son John will later become an anti-Skynet resistance leader. But Reese winds up fathering John. His time travel was a part of the timeline all along.

This idea — that time travel could be possible, but must be consistent with the past as it has already taken place — is known among physicists as the Novikov self-consistency principle. It's possible that this principle is wrong, but it behooves people who think past-altering time travel is possible to explain how to avoid paradoxes. Take, for instance, the most famous time travel problem, the grandfather paradox: Suppose you go back in time and kill your grandfather before your mother/father has been conceived. This action creates a world in which you exist but your existence is logically impossible. Things like that just can't happen, which is why many physicists and philosophers embrace the Novikov self-consistency principle. The late, great philosopher David Kellogg Lewis explained this well in his 1976 paper "The Paradoxes of Time Travel":

If Tim did not kill Grandfather in the "original" 1921, then if he does kill Grandfather in the "new" 1921, he must both kill and not kill Grandfather in 1921—in the one and only 1921, which is both the "new" and the "original" 1921. It is logically impossible that Tim should change the past by killing Grandfather in 1921. So Tim cannot kill Grandfather.

There are some fictional depictions of attempted retroactive Hitler assassinations that explain how this principle works in practice. In "Cradle of Darkness," an episode of the 2002-'03 reboot of the Twilight Zone, Katherine Heigl's character is sent back in time to kill baby Hitler. She succeeds — but Hitler's mother adopts another baby and raises it as Adolf, who grows up to lead the Nazi Party, start World War II, carry out the Holocaust, etc.

Somewhat similarly, Eric Norden's 1977 novella The Primal Solution imagines an elderly Jewish scientist and Holocaust survivor attempting to go back in time, control Hitler's mind, and force him to drown himself. Hitler survives, identifies the force trying to kill him as Jewish, and becomes a vociferous anti-Semite, setting the Nazi rise to power and the Holocaust into motion.

These aren't very satisfying versions of Hitler-killing time travel. But they're versions that obey the Novikov self-consistency principle, and thus make considerably more internal sense than versions in which you really can go back in time and kill the actual Hitler.

Can we have any sense of what the ramifications of killing Hitler would be?

Okay, so time traveling metaphysics are tricky. Let's get ourselves out of this thicket, then, by supposing instead that no time traveling takes place and instead we're an Austrian living in the town of Braunau am Inn in 1889 who has a strong premonition that the wee baby Adolf is going to grow up to kill tens of millions of people, and we are thus driven to kill him. You know your predictions are correct. No time travel paradoxes are going to be created. Do you do it?

Well … maybe? It depends on quite a few factors. For one thing, baked into the premise of this question is the idea that the Nazis would not have risen to power, launched World War II, and carried out the Holocaust were it not for the existence of Adolf Hitler. You could certainly imagine a history in which Hitler didn't exist, the party lacked a charismatic leader, and it never came to power. Germany muddled through the Great Depression, the Weimar Republic kept going, and World War II never arose.

But you can also imagine a history in which another leader emerged who was even more effective than Hitler. This is the story offered by the comedian Stephen Fry in his novel Making History. In it, a history graduate student named Michael Young goes back in time and renders Hitler's father infertile. However, the Nazi Party still takes power, under a leader named Rudolf Gloder who, lacking Hitler's personal character flaws, is able to acquire nuclear weapons, obliterate Moscow and St. Petersburg, conquer almost all of Europe permanently, exterminate the continent's Jewish population, and carry on a cold war with the US indefinitely.

You could also imagine an alternate history where the Nazis don't take power but the Völkisch movement in post-WWI Germany gives rise to another virulently anti-Semitic regime, or at least a regime that also sparks a second world war. Or maybe Germany is fine, but absent WWII, tensions between the US and the Soviet boil into a hot war that is even bloodier and more destructive than the actual Second World War was. Maybe this war doesn't lead to the kind of postwar human rights revolution that WWII actually did, slowing the spread of liberal democracy and causing additional suffering for millions.

All of which is to say: We have no idea how the world would have differed if Hitler had died in infancy. We don't know how much weaker, or stronger, the Nazi Party would've been. We don't know if a second UK/France/Russia versus Germany war could've been avoided and, if so, whether another, bloodier war would've occurred instead. And unless we have answers on that, we can't know the consequences of killing Hitler and thus whether killing Hitler did more good than harm.

If we can't know if killing Hitler is right or wrong, can we know if anything is right or wrong?

This is actually a general problem for consequentialist moral theories — that is, theories where the morality of an action depends entirely on what the consequences of that action would be. In his classic 2000 paper "Consequentialism and Cluelessness," the University of Sheffield's James Lenman explained the issue using the case of a German bandit in the year 100 BCE attempting to decide whether to kill a distant ancestor of — you guessed it — Adolf Hitler:

Imagine we are in what is now southern Germany a hundred years before the birth of Jesus. A certain bandit, Richard, quite lost to history, has raided a village and killed all its inhabitants bar one.This final survivor, a pregnant woman named Angie, he finds hiding in a house about to be burned. On a whim of compassion, he orders that her life be spared. But perhaps, by consequentialist standards, he should not have done so. For let us suppose Angie was a great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great grandmother of Adolf Hitler. The millions of Hitler's victims were thus also victims of Richard's sparing of Angie. … Do Hitler's crimes mean that Richard acted wrongly, in consequentialist terms? They do not. For Hitler's crimes may not be the most significant consequence of Richard's action. Perhaps, had Richard killed Angie, her son Peter would have avenged her, thus causing Richard's widowed wife Samantha to get married again to Francis. And perhaps had all this happened Francis and Samantha would have had a descendant 115 generations on, Malcolm the Truly Appalling, who would have conquered the world and in doing so committed crimes vastly more extensive and terrible than those of Hitler.

Lenman's point is that if morality is really about maximizing good consequences, then it's a problem that the immediate consequences of an action like killing an innocent villager can be swamped by the consequences thousands of years in the future, which no one could ever reasonably foresee. Maybe I met someone at a bar last weekend whose progeny 2,000 years from now will cause human extinction. That would imply that the worst thing I ever did in my entire life was refrain from murdering that bar acquaintance. This seems to imply that there's basically no way to know if you're making the right ethical decisions. Either ethical living is impossible, or a moral theory less dependent on actions' consequences is needed.

I'm less pessimistic than Lenman is. As Tyler Cowen argued in a response paper, the existence of serious uncertainty is important, and humbling, but doesn't render estimation of an event's likely effects totally impossible. If we know the near-term effects of foiling a nuclear terrorism plot are that millions of people don't die, and don't know what the long-term effects will be, that's still a good reason to foil the plot.

But the "epistemic critique," as Lenman's argument has come to be known, is important when we're considering less direct consequences, like the effects of murdering an infant in 1889 on war and peace in 1939. This isn't like foiling a nuclear bomb plot. We just don't know what the consequences of killing Hitler would've been at any point. That renders the problem of Hitler infanticide all but unsolvable.

Sign up for the Future Perfect newsletter. Twice a week, you’ll get a roundup of ideas and solutions for tackling our biggest challenges: improving public health, decreasing human and animal suffering, easing catastrophic risks, and — to put it simply — getting better at doing good.