Thomas Kuhn was the most famous of the philosophers who have examined the processes by which people adopt a new theory, model, or way of knowing. His works focused on how scientists adopt a new theory, but his conclusions can be applied to all human thinking. His most famous book proposes that all our ways of knowing, even our most cherished ones, are tentative and arbitrary.² 

Under his model of how human knowledge grows, humans advance from an obsolete idea to a newer, more comprehensive one by paradigm shifts— that is by leaps and starts rather than in a steady march of gradually growing enlightenment. We “get”, and then start to think under, a new model or theory by a kind of conversion experience, not by a gradual process of persuasion and growing understanding.

Caution and vigilance seem to be the only rational attitudes to take under such a view of the universe and the human place in it. To many people, the idea that all of the mind’s systems—and its systems for organizing systems and perhaps even for its overriding operating system, its sanity—are tentative and are subject to constant revision seems not just disturbing; it seems absurd. But then again, cognitive dissonance theory would lead us to predict that humans would quickly dismiss such a scary picture of themselves. We don’t like to see ourselves as lacking in any unshakable principles or beliefs. However, evidence and experience suggest we are indeed almost completely lacking in fixed concepts or beliefs, and we do nearly always evolve personally and collectively in those scary ways. (Why I say nearly always and almost completely will become clear shortly.)

Now, at this point in the discussion, opponents of Bayesianism begin to marshal their forces. Critics of Bayesianism give several varied reasons for continuing to disagree with the Bayesian model, but I want to deal with just two of the most telling—one is practical and evidence-based, and the other, which I’ll discuss in the next chapter, is purely theoretical.

In the first place, say the critics, Bayesianism simply can’t be an accurate model of how humans think because humans violate Bayesian principles of rationality every day. Every day, we commit acts that are at odds with what both reasoning and experience have shown us is rational. Some societies still execute criminals. Men continue to bully and exploit, even beat, women. Some adults still spank children. We fear people who look different from us on no other grounds than that they look different from us. We shun them even when we have evidence showing there are many trustworthy individuals in that other group and many untrustworthy ones in the group of people who look like us. We do these things even when research indicates that such behaviors and beliefs are counterproductive. Their effects are opposite to what the doers of the actions originally intended. 

Over and over, we act in ways that are illogical by Bayesianism’s own standards. We stake the best of our human and material resources on ways of behaving that both reasoning and evidence say are not likely to work. Can Bayesianism account for these glaring bits of evidence that are inconsistent with its model of human thinking?