  It is curious to learn that Enrico Fermi knew how to base probabilistic
inference on Bayes theorem, and that some influential notes on statistics for
physicists stem from what the author calls elsewhere, but never in these notes,
{\it the Bayes Theorem of Fermi}. The fact is curious because the large
majority of living physicists, educated in the second half of last century -- a
kind of middle age in the statistical reasoning -- never heard of Bayes theorem
during their studies, though they have been constantly using an intuitive
reasoning quite Bayesian in spirit. This paper is based on recollections and
notes by Jay Orear and on Gauss' ``Theoria motus corporum coelestium'', being
the {\it Princeps mathematicorum} remembered by Orear as source of Fermi's
Bayesian reasoning.
