That what is self-evident to one person can be seen as silly by another should give us pause about the reliability of common sense as a basis for understanding the world. How can we be confident that what we believe is right when someone else feels equally strongly that it’s wrong—especially when we can’t articulate why we think we’re right in the first place? Of course, we can always write them off as crazy or ignorant or something and therefore not worth paying attention to. But once you go down that road, it gets increasingly hard to account for why we ourselves believe what we do. Consider, for example, that since 1996 support among the general public for allowing same-sex couples to marry has almost doubled, from 25 percent to 45 percent. Presumably those of us who changed our minds over this period do not think that we were crazy fourteen years ago, but we obviously think that we were wrong. So if something that seemed so obvious turned out to be wrong, what else that we believe to be self-evident now will seem wrong to us in the future?

Once we start to examine our own beliefs, in fact, it becomes increasingly unclear even how the various beliefs we espouse at any given time fit together. Most people, for example, consider their own views about politics to be derived from a single coherent worldview: “I’m a moderate liberal” or “I’m a diehard conservative,” and so on. If that were true, however, then one would expect that people who identify as liberals would tend to espouse the “liberal” perspective on most matters, and that conservatives would espouse a consistently different view. Yet research finds that regardless of whether people identify themselves as liberals or conservatives, what they think about any one issue, like, say, abortion, has relatively little relation to what they believe about other issues, such as the death penalty or illegal immigration. In other words, we have the impression that our particular beliefs are all derived from some overarching philosophy, but the reality is that we arrive at them quite independently, and often haphazardly.

The same difficulty of reconciling what, individually, appear to be self-evident beliefs shows up even more clearly in the aphorisms that we invoke to make sense of the world. As sociologists are fond of pointing out, many of these aphorisms appear to be direct contradictions of each other. Birds of a feather flock together, but opposites attract. Absence indeed makes the heart grow fonder, but out of sight is out of mind. Look before you leap, but he who hesitates is lost. Of course, it is not necessarily the case that these beliefs are contradictory — because we invoke different aphorisms in different circumstances. But because we never specify the conditions under which one aphorism applies versus another, we have no way of describing what it is that we really think or why we think it. Common sense, in other words, is not so much a worldview as a grab bag of logically inconsistent, often contradictory beliefs, each of which seems right at the time but carries no guarantee of being right any other time.

This is an extract from Duncan J Watts' Everything Is Obvious published by Crown Business