The Stone is a forum for contemporary philosophers and other thinkers on issues both timely and timeless.

The Stone is featuring occasional posts by Gary Gutting, a professor of philosophy at the University of Notre Dame, that apply critical thinking to information and events that have appeared in the news.

The probability that there is intelligent life somewhere other than earth increases as we discover more and more solar systems that seem capable of sustaining life. The thought that there might be extraterrestrial intelligences (ETI) somewhere out there excites us and has led to organized efforts to contact any such beings. We have sent space probes with data about us, and we transmit signals with a structured content (like symbols expressing mathematical formulae) to what we hope will be an intergalactic audience. The search for extraterrestrial intelligence project (SETI) is obviously based on the assumption that the possible benefits of contact with ETI outweigh the possible harms. But do they?

A recent study by researchers at Penn State and NASA provides a useful outline of the various ways that encounters with ETI could be beneficial, neutral or harmful to us. The study faces up to the most chilling possibilities: ETI might “eat us, enslave us, attack us,” inadvertently infect us with horrible diseases or just decide to eliminate us for the greater good of the universe. (Regarding this last point, the report is especially concerned that ETI might be at least metaphorically green and see us a threat to the universe’s ecology.)

The report draws no conclusions about the wisdom of pursuing SETI, though it does urge the need to develop quantitative measures of possible harms and benefits. Its final sentence seems content with the idea that we will “continue the search for extraterrestrials into the future.” Especially after reading the report, I am not so content.

What is likely to happen if we make contact with ETI? Given the size of astronomical distances and assuming the speed of light as the maximum possible velocity, the most likely outcome is not real contact but merely an exchange of messages, perhaps at very long intervals. Little chance of harm there.

But there is still non-zero probability of real contact. Since we have no way of predicting with any certainty the outcome of such contact, it might seem that we have no reason to assume a bad rather than a good result. From this we might conclude that there is no objection to pursuing SETI, if only to satisfy our curiosity.

But we do know this: for the foreseeable future, contact with ETI would have to result from their coming here, which would in all likelihood mean that they far surpassed us technologically. They would be able to enslave us, hunt us as prey, torture us as objects of scientific experiments, or even exterminate us and leave no trace of our civilization. They would, in other words, be able to treat us as we treat animals — or as our technologically more advanced societies have often treated less advanced ones.

This suggests an argument against SETI that is the reverse of Pascal’s famous wager argument for believing in God. Pascal’s idea was that even a small probability of bringing about an enormous good (without risking unacceptable evil) was good reason for acting. This is a reasonable principle: even a small prospect of enormous good can swamp the prospect of more probable but much lesser goods. Pascal’s argument runs into trouble not because of this principle but because of worries about, for example, which God we ought to believe in. (There is also, as William James pointed out, the disconcerting possibility that God might be particularly ill-disposed to people who believe in him through the calculating reasoning of the wager argument.)

The swamping principle also applies to a small possibility of an enormous evil, which can provide a good reason for not acting. This would seem to be the case with ETI. Since there’s at least a small (and perhaps a not so small) probability that they will bring us catastrophic evil, why should we risk such an outcome?

One reason might be that ETI could instead bring us enormous benefits: they might even lead us to a paradise of peace, wisdom and joy. But there is no reason to think that such a paradise is more probable than a hell of slavery or extermination. And enormous gains are not worth the equal risk of horrendous loss. Who would take a bet that promised, at equal odds, either a lifetime of unalloyed happiness or a lifetime of utter misery? Better to stick with the likelihood of a normal human life, mixed with joy and sorrow.

Another possible reason is that ETI might in fact save us from horrors equal to the worst they might inflict on us. If the probability of their saving us were equal to or greater than of their destroying us, then the bet of making contact might well be worth it. Here the most plausible suggestion is that, without intervention from powerful and good ETI, we are likely to destroy ourselves through nuclear war. But there is a failure of imagination in thinking that nuclear annihilation is as bad as the worse that ETI might do to us. They might, for example, give us each thousands of years of excruciatingly painful existence as their slaves. This might not even be due to moral perversity; they might be so beyond us that they were incapable of recognizing us as objects of moral concern.

We cannot know what might happen to us from contact with ETI. But we do know that there may well be unthinkably horrible outcomes that are not likely to be offset by potential benefits. We should not take the SETI bet.