Lately my grandson’s interest in the Marvel and DC comics world of superheroes has been turning his mind to the conundrums of time travel and the intricacies of rewriting the past.

My own pop-culture exposure to time-travel plotlines dates from the Back to the Future movies and Men in Black, both of which firmly engraved in my mind this cardinal rule of: Never, no matter how seemingly urgent the necessity or noble the motivation, interfere to change anything in the past. Introducing even the smallest anachronism or attempting the most trivial improvement inevitably produces unintended and undesirable results. In fact, we can attain no realistic understanding of the manifold ways in which even a seemingly minor event can alter the present and future.

My grandson knows all this too, but he (and his comic-book superheroes) cannot resist indulging in this temporal temptation. He wrestles with the constraints that would keep him from righting wrongs on a grand scale, unconfined by space or time (of course, he also lacks superpowers and a means of traveling to the past, but one challenge at a time).

But, begging their pardon, the sci-fi and superhero writers should know enough by now to ignore the itch to bend the rules for a higher purpose. As a mind game, contemplating time travel can be thought-provoking, but the thoughts it provokes should be emphasizing just how very little we human beings know—how very little we can ever know—about the tangled effects of the workings of billions of human wills in our little pocket of a vast and eons-old universe that did not make itself.

Oh sure, our technology develops dizzyingly greater capabilities, and we talk a good game about artificial intelligence. Our ability to interfere with whatever is occurring in the present also increases—at times encouragingly, at other times disquietingly. Even the most scientifically interventionist are quite able to come up with examples of the latter—such as global warming or depletion of the earth and extinction of the species. For these as for other ills, however, they tend to turn to systematic, global solutions—grand schemes—rather than immediate rights and wrongs cascading from small discrete actions.

But the unappreciated utility of the time-travel experiment—the truth it presents those grappling with its complexities—is comprehension of our clear incapacity to predict most (in fact, almost all) of the repercussions of our actions. These inherent limitations are clear even when we are trying to positively influence our present and our future. That is, they should be clear, but the mindset that allows people to think they can helpfully meddle with past events seems allied with a predisposition to let the ends justify the means.

And why should this not be their attitude, when most of our moral public discourse and much of our internal moral decision-making, when it is not frankly self-interested or merely sentimental, is conducted along roughly utilitarian (the greatest good for the greatest number) lines?

Abortion is one of the most robust examples of this. On the rational level, for 45 years legalized abortion has been mentoring American women to decide whether to keep their unborn, unplanned babies by considering whether they want the child, whether they can afford the child, whether it is good for their family and career. It is not that the factors balanced against the life of their child are slight things—often only when balanced against the life of a human being can legitimate concerns such as rejection by family, desertion by a spouse, homelessness, poverty, the demands of providing for other children, come up short. But the balancing act itself, in a clear-cut matter of life and death, is itself suspect.

And that leads us back to space/time travel and the near-overwhelming desire to intervene to alter an unpleasant or even appalling destiny. The interfering time traveler observes a family member’s death or defeat or dishonor and recognizes it as an evil, a sorrow, and a precipitator of unpleasant fallout on others in the future. If he has not seen other time-travel movies or if he is blinded to common sense by his desire to change the past, he will try to change the poor choice or the unfortunate intersection of lives that, in his judgment, has caused only great unhappiness. But there is so much we don’t know—even about the past and the present (experienced so differently and largely so hiddenly by so many people), let alone about the future. We can be wrong even about the needs, desires, dreams, and capacities of those in our very limited sphere of vision, let alone the vast majority of people outside it.

Is this a counsel of despair over the possibility of successfully doing good? No, it is perhaps the reverse. Rather than exalting a kind of passive model of “Do nothing, so you will do no harm” (itself a hopeless injunction, since doing nothing is also a kind of doing), it would be better to do good and avoid evil not measured by five-year-plans or reduced human footprints or population control or self-fulfillment or romantic love or career tracks or political success or business efficiency, but in the classic sense of doing acts that are good in themselves and refraining from doing those that are evil. Not only is this the most morally defensible way of living, but it seems to me also to be the most practical. It is based on what we can know.

But some are not so sure of that last assertion. In fact, the immediate objection will likely be, “How do we determine what is good and bad except by considering the repercussions of our actions?” There are several tracks along which one could respond, including the religiously teleological one that identifies ultimate goals and aims for which doing the good God has commanded and avoiding the evil He has forbidden are the means. (Such ultimate goals could, for example, be doing the will of God or perhaps attaining heaven.)

But, less divisively for a pluralist society and quite as legitimately, there is an easier way for people to discriminate between most of the good and bad choices in their daily lives—and certainly between the most serious good and bad choices. That is by looking at the choices we try to talk ourselves out of when we bring  in the “means to an end” scale of comparison. There we find the moral goods, the direct and immediately perceivable ends of our acts, that tether us in ways that we then try to break free from.

So for example, if you are contemplating leaving your wife and children because you have fallen in love with someone else, you likely will be telling yourself something like this: “I know I promised ‘til death do we part,’ and of course the children won’t see as much of me, but I will be so unhappy not being with the person I really love—and she will be unhappy too! Life is short, and we have a right to pursue happiness. Even for my wife, it’s better to just admit that I made a mistake or that we have grown apart, so she is free to find someone else.”

In this case, abandoning the means-to-an-end justification for our actions means looking at the moral duty the husband was dismissing by bringing up his own happiness (and that of his beloved and, much more distantly and hypothetically, that of his wife if she found a new love in the future). What’s left—what would keep the husband bound to his wife—is the marriage vow.

This pattern is repeated across a great many moral decisions that people often make with the understanding that the end justifies the means. The examples can come from the political right or left or from outside politics: that polluting or despoiling the environment or treating farm animals inhumanely is okay if it is a means to the end of providing cheap food or making inexpensive consumer goods widely available or keeping a labor force employed; that bombing civilians in wartime is acceptable to achieve the end of peace or lower total casualties; that euthanasia is acceptable to cut short a now-meaningless existence, prevent pain, or even perhaps prevent financial hardship for those who had to care for the person; that cheating on an exam is acceptable because the question was unfair or everybody else is doing it or I need to do well to get ahead. Whatever moral good the “because” clause is meant to trump determines the good act one should perform or the bad act one should avoid doing.

Of course, a great many choices still lie outside this framework—even choices that have a moral component, such as many voting matters, decisions about where to live, what to do for a living, whom to marry, when to have children and how many, how to bring them up, what to give to charity and which charities to contribute to, how to allocate leisure time. These decisions are largely prudential—they are largely about how best to go about achieving an already recognized good. Once clearly wrong actions have been eliminated from consideration, they leave a number of morally acceptable outcomes to be mulled over. And at times there are truly competing moral duties (rather than a good that perhaps one can pursue if it does not collide with a moral duty). Some of these require complicated moral casuistry to determine what is the right choice. Some end-of-life medical-treatment questions fall into this category, or the famous “principle of double effect” applied to a pregnancy that truly threatens the life of the mother. But very few of the decisions we make, even on important matters, are like that.

The time-travel mental experiment casts into clear relief how very little we do and can know about even relatively direct effects of our actions over relatively brief durations of time, let alone over continents or millennia. We are in time, we are in space, we are small and finite and not self-made—of course we can’t see anything like the whole picture, though we can make our own relatively small-scale plans, build governments, set the boundaries of nations, invent labor-saving gadgets, come up with the internet, and the like. How then to act, day in and day out, when facing moral decisions whose ultimate effects are far beyond our very short-sighted view? Do good and avoid evil, as best we can, and leave the rapidly extending ripples of our actions to the providence of God, who is the Master of Time rather than its subject.