Together with Frank Ramsey, the Italian “radical probabilist” Bruno de Finetti is widely considered to be the main progenitor and promoter of the idea that probability is inherently subjective. According to this view, all we can do is specify our prior beliefs and then ensure that they remain coherent, that is, free from internal inconsistencies. And the only way to ensure such coherence is to update those beliefs in light of new data through the use of Bayes’ rule.

Dennis Lindley once stated that a decent study of de Finetti would take a statistician one or two years (but that it would be worth the investment). Recently I decided to bite the bullet and order the reprint of de Finetti’s standard work “Theory of Probability”. After browsing the book I must say that it looks much less daunting than I had anticipated; perhaps this is because I have already accepted the main Bayesian premise, or because I am used to read work by Harold Jeffreys. At any rate, de Finetti’s writing is clear and lively, and I look forward to studying its contents in more detail.

My main disappointment was that the preprint of de Finetti’s book concerns the 1970 version, and this means that the famous preface to the 1974 edition is missing. In a future post I intend to provide an annotated version of that preface, but here I just give its most iconic statement to whet the appetite:

“My thesis, paradoxically, and a little provocatively, but nonetheless genuinely, is simply this:


    The abandonment of superstitious beliefs about the existence of Phlogiston, the Cosmic Ether, Absolute Space and Time,…, or Fairies and Witches, was an essential step along the road to scientific thinking. Probability, too, if regarded as something endowed with some kind of objective existence, is no less a misleading misconception, an illusory attempt to exteriorize or materialize our true probabilistic beliefs.” (de Finetti, 1974, p. x)


de Finetti, B. (1974). Theory of Probability, Vol. 1 and 2. New York: John Wiley & Sons.

Galavotti, M. C. (Ed.) (2009). Bruno de Finetti: Radical Probabilist. London: College Publications.

About The Author

Eric-Jan Wagenmakers

Eric-Jan (EJ) Wagenmakers is professor at the Psychological Methods Group at the University of Amsterdam.