Chapter 5 – Bayesianism: How It Works

                                                                Thomas Bayes.

The best answer to the problem of what human minds and human knowing are is that we are really all Bayesians. On Bayesianism, I can build a universal moral system. So what is Bayesianism?

Thomas Bayes was an English Presbyterian minister, statistician, and philosopher who formulated a specific theorem that is named after him: Bayes’ theorem. His theory of how humans form tentative beliefs and gradually turn those beliefs into concepts has been given several mathematical formulations, but in essence it says a fairly simple thing. We tend to become more convinced of the truth of a theory or model of reality the more we keep encountering bits of evidence that, first, support the theory and, second, can’t be explained by any of the competing models of reality that our minds already hold. (A fairly accessible explanation of Bayes‘ theorem is on the Cornell University Math Department website.¹)

Under the Bayesian view, we never claim to know anything for certain. We simply hold most firmly a few beliefs that we consider very highly probable, and we use them as we make decisions in our lives. We then assign to our other, more peripheral beliefs, lesser degrees of probability, and we constantly track the evidence supporting or disconfirming all of our beliefs. 

We accept as given that all beliefs, at every level of generality, need constant review and updating, even the ones that seem for long periods to be working well at guiding us in handling real life.

The more that a new theory enables a mind to establish coherence within its whole conceptual system and all its sets of sense-data memories, the more persuasive the theory seems. If the evidence favouring the theory mounts, and its degree of consistency with the rest of the beliefs and memories in the mind also grows, then finally, in a leap of understanding, the mind promotes the theory up to the status of a concept and incorporates the new concept into its total stock of thinking machinery.

At the same time, the mind nearly always has to demote to inactive status some formerly held beliefs and concepts that are not commensurable with the new concept and so are judged to be less efficient in enabling the mind to organize and use its total stock of memories. This is especially true of all mental activities involved in the kinds of thinking that are now being covered by the new model or theory. For example, if you absorb and accept a new theory about how your immune system works, that idea, that concept, will inform every health-related decision you make thereafter.