Bayes' theorem, Spinoza, Zen and the art of fortune telling.

As those who have spent any length of time with me and shown the slightest inclination to seem at all interested will know, I am a confirmed Bayesian. To those of you who haven't read chapter three of my thesis, or yet become acquainted with one of the most interesting and philosophically rewarding aspects of modern statistics, Bayes' Theorem is essentially a restatement of the law of conditional probability, i.e. P(A given B) = P(B given A) * P(A) / P(B).

"What does that have to do with fortune telling?", I hear you ask (since I can tell everyone reading is utterly hooked by now). Well lets introduce one more piece into the equation, with P(A|B,I) meaning "the probability of A given B and initial information I". So P(A|B,I) = P(B|A,I) * P(A|I) / P(B|I).

Now let's suppose A is "the future" and B is "the state of these fresh chicken giblets" and I is my state of knowledge of the world before sacrificing the hapless bird, this tells me that regardless of how informative the exact configuration of the entrails laid out on the altar before me are, my knowledge of how the world works will play a role in the prediction I give to the assembled masses. Of course the more useless are the innards, the more important are the views I held before I looked at them.

This brings us to Spinoza. According to Antony Damasio's fascinating book "Looking for Spinoza", the heretic philosopher of Portuguese Jewish origin had intuitively grasped what neuroscientists are only now coming to understand. This was that the mind and body are intimately connected (as opposed to Descarte's view), and the former is largely a model of the latter. Not only that, but our brains contain a depiction of the world around us and its interaction with ourselves, built up from our senses; our emotions and feelings conspire to push us in the direction which will best lead to our material well-being and to the propagation of the species (our spiritual and mental health is thus intimately connected with our physiological health and the social and physical environment in which we find ourselves - a fact which has considerable political implications).

The old adage "the truth will set you free" makes perfect sense in this framework - if our internal model of the world, including our own and others' motivations and states of mind, corresponds to that which best fits the information acquired by our senses and experience (i.e. it is as close to the truth as is possible with the information at our disposal), then we are at peace. Each person's individual "truth" may be different however, even when placed in the same situation, as there is never enough information available to uniquely determine the state of the world at any one time, and every person's background is different. However, the more shared experiences they have, and perhaps the closer their temperament, the more similar will be their vision of the world. (Obviously someone with access to more and better information will have a more accurate representation of the world as it really is, and be capable of making better decisions.)

On top of this, our brains are capable of projecting our future happiness, based on the model of how the world works we have built up from our experiences and on the conscious plans we may have for the future - so thinking positively is worth it! The upshot of all this is that our brains contain a considerable amount of information which may not be consciously available to us, yet guides our actions often without us being aware of what is happening. So-called "gut feelings" are well worth paying heed to, though care should be taken if you are in unfamiliar situations, or especially when dealing with things which were not present during human evolution, such as weblogs (yes I know you feel a sudden urge to close your eyes and fall asleep, but the future of mankind depends on you to continue reading this!).

Back to the subject of Japan, and zen...

For a hundred yen or so, it is possible to buy a small piece of paper with a prediction of your fortune on it at most respectable temples. Being wily sorts and with a good few thousand years of study of human nature behind them, the purveyors of these things give you a good choice of subjects at the same time, from travel plans to marriage. Any decent crystal ball reader worth her caravan will also keep you occupied with a similar plethora of advice, until she spies in your eyes something that hits home.

So my contention is that while the predictions on these pieces of paper may indeed be entirely random, sometimes they seem eerily significant (such as "Work: you must work hard or you will fail"). This happens because they probe your Bayesian priors, as the initial information "I" is known in the trade. Often these are hidden from conscious view, but they represent your (limited but useful) experience of the world and your knowledge of yourself. So when Mystic Meg's pronouncement that you should "beware horseless carriages" has an inexplicable ring of truth to it, your car was probably making funny noises yesterday and you should check the oil.