Common sense is exquisitely adapted to handling the kind of complexity that arises in everyday situations. And for those situations, it's every bit as good as advertised. But "situations" involving corporations, cultures, markets, nation-states, and global institutions exhibit a very different kind of complexity from everyday situations. And under these circumstances, common sense turns out to suffer from a number of errors that systematically mislead us. Yet because of the way we learn from experience--even experiences that are never repeated, or that take place in other times and places--the failings of commonsense reasoning are rarely apparent to us. Rather, they manifest themselves to us simply as "things we didn't know at the time" but which seem obvious in hindsight. The paradox of common sense, therefore, is that even as it helps us make sense of the world, it can actively undermine our ability to understand it.

Common sense 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. The fragmented, inconsistent, and even self-contradictory nature of common sense does not generally present a problem in our everyday lives. The reason is that everyday life is effectively broken up into small problems, grounded in very specific contexts that we can solve more or less independently of one another. Under these circumstances, being able to connect our thought processes in a logical manner isn't really the point. It doesn't really matter that absence makes the heart grow fonder in one situation, and that out of sight is out of mind in the next. In any given situation we know the point we are trying to make, or the decision we want to support, and we choose the appropriate piece of commonsense wisdom to apply to it.

If we had to explain how all our explanations, attitudes, and commonsense beliefs fit together, we would encounter all kinds of inconsistencies and contradictions. But because our experience of life rarely forces us to perform this task, it doesn't really matter how difficult it would be. Where it does start to matter, however, is when we use common sense to solve problems that are not grounded in the immediate here and now of everyday life--problems that involve anticipating or managing the behavior of large numbers of people, in situations that are distant from us either in time or space. At some level we understand that the world is complicated, and that everything is somehow connected to everything else. But when we read some story about reforming the healthcare system, or about banker bonuses, or about the Israel-Palestine conflict, we don't try to understand how all these different problems fit together. We just focus on the one little piece of the underlying tapestry of the world that's being presented to us at that moment, and form our opinion accordingly. In this way, we can flip through the newspaper while drinking our morning cup of coffee and develop twenty different opinions about twenty different topics without breaking a sweat. It's all just common sense.

When individuals are influenced by what other people are doing, similar groups of people can end up behaving in very different ways. This may not sound like a big deal, but it fundamentally undermines the kind of commonsense explanations that we offer for why some things succeed and others fail, why social norms dictate that we do some things and not others, or even why we believe what we believe. Commonsense explanations sidestep the whole problem of how individual choices aggregate to collective behavior simply by replacing the collective with a representative individual. And because we think we know why individual people do what they do, as soon as we know what happened, we can always claim that it was what this fictitious individual--"the people," "the market," whatever--wanted. Just as you can know everything about the behavior of individual neurons and still be mystified by the emergence of consciousness in the human brain, so too you could know everything about individuals in a given population--their likes, dislikes, experiences, attitudes, beliefs, hopes, and dreams--and still not be able to predict much about their collective behavior. To explain the outcome of some social process in terms of the preferences of some fictitious representative individual therefore greatly exaggerates our ability to isolate cause and effect.

When we hear about a large forest fire, of course, we don't think that there must have been anything special about the spark that started it. Indeed, such an idea would be laughable. Yet when we see something special happen in the social world, we are instantly drawn to the idea that whoever started it must have been special also. And of course, whenever a large cascade did take place in our simulations, it was necessarily the case that someone had to have started it. However unexceptional that person might have seemed in advance, in retrospect they would seem to fit exactly the description of Malcolm Gladwell's law of the few: the "tiny percentage of people who do the majority of the work." What we knew from our simulations, however, was that there really was nothing special about these individuals--because we had created them that way. The majority of the work was being done not by a tiny percentage of people who acted as the triggers, but rather by the much larger critical mass of easily influenced people. What we concluded, therefore, is that the kind of influential person whose energy and connections can turn your book into a bestseller or your product into a hit is most likely an accident of timing and circumstances. An "accidental influential" as it were.

Creeping determinism means that we pay less attention than we should to the things that don't happen. The absence of "counterfactual" versions of history leads us to tend to perceive what actually happened as having been inevitable. The tendency is related to the better-known phenomenon of hindsight bias, the after-the-fact tendency to think that we "knew it all along." Together, creeping determinism and sampling bias lead commonsense explanations to suffer from what is called the post-hoc fallacy. The fallacy is related to a fundamental requirement of cause and effect--that in order for A to be said to cause B, A must precede B in time. But just because B follows A doesn't mean that A has caused B. It's an obvious point, and in the physical world we have good enough theories about how things work that we can usually sort plausible from implausible. But when it comes to social phenomena, common sense is extremely good at making all sorts of potential causes seem plausible. The result is that we are tempted to infer a cause-and-effect relationship when all we have witnessed is a sequence of events. This is the post-hoc fallacy.

Common sense and history conspire to to generate the illusion of cause and effect where none exists. On the one hand, common sense excels in generating plausible causes, whether special people, or special attributes, or special circumstances. And on the other hand, history obligingly discards most of the evidence, leaving only a single thread of events to explain. Commonsense explanations therefore seem to tell us why something happened when in fact all they're doing is describing what happened.

Just as commonsense explanations of the past confuse stories with theories, so too does commonsense intuition about the future tend to conflate predictions with prophecies. When we look to the past, we see only the things that happened--not all the things that might have happened but didn't--and as a result, our commonsense explanations often mistake for cause and effect what is really just a sequence of events. Correspondingly, when we think about the future, we imagine it to be a unique thread of events that simply hasn't been revealed to us yet. In reality no such thread exists--rather, the future is more like a bundle of possible threads, each of which is assigned some probability of being drawn, where the best we can manage is to estimate the probabilities of the different threads. But because we know that at some point in the future, all these probabilities will have collapsed onto a single thread, we naturally want to focus on the one thread that will actually matter. Likewise, when we look to the past, we do not feel any confusion about what we mean by the "events" that happened, nor does it seem difficult to say which of these events were important. And just as the uniqueness of the past causes us to think of the future as unique as well, so too does the apparent obviousness of past events tempt us into thinking that we ought to be able to anticipate which events will be important in the future. Yet what these commonsense notions overlook is that this view of the past is a product of a collective storytelling effort--not only by professional historians but also by journalists, experts, political leaders, and other shapers of public opinion--the goal of which is to make sense of "what happened." Only once this story has been completed and agreed upon can we say what the relevant events were, or which were the most important. Thus it follows that predicting the importance of events requires predicting not just the events themselves but also the outcome of the social process that makes sense of them.