**Example 1:**- ``What is the probability
that a molecule of nitrogen at room
temperature has a velocity between 400 and 500 m/s?''. The answer
appears easy: ``take the Maxwell distribution formula from a textbook,
calculate an integral and get a number''. Now
let us change the question:
*``I give*__You__a vessel containing nitrogen and a detector*capable of measuring the speed of a single molecule and*. Anybody who has minimal experience (direct or indirect) of experiments would hesitate before answering. He would study the problem carefully and perform preliminary measurements and checks. Finally he would__You__set up the apparatus (or__You__let a person__You__trust do it). Now, what is the probability that the__first__molecule that hits the detector has a velocity between 400 and 500 m/s?''*probably*give not just a single number, but a range of possible numbers compatible with the formulation of the problem. Then he starts the experiment and eventually, after 10 measurements, he may form a different opinion about the outcome of the eleventh measurement. **Example 2:**- ``What is the probability that the gravitational constant
has a value between
and
mkgs?''. Before 1994 you
could have looked at the latest
issue of the Particle Data Book[33]
and answered that the probability was . Since then -- as you
probably know -- three new measurements of have been
performed[34]
and we now have
__four__numbers which do not agree with each other (see Tab. ). The probability of the true value of being in that range is currently dramatically decreased.

**Table:**Results of measurements of the gravitational constant .Institute (ppm) () CODATA 1986 (``'') 128 - PTB (Germany) 1994 83 MSL (New Zealand) 1994 95 Uni-Wuppertal 105 (Germany) 1995

**Example 3:**- ``What is the probability that the mass of the top
quark, or that of any of the supersymmetric particles, is below
20 or
GeV?''. Currently it looks
as if it must be zero. Ten years ago
many experiments were intensively looking
for these particles in those energy ranges.
Because so
many people where searching for them, with
enormous human and capital investment, it meant that,
__at that time__, the probability was considered rather__high__: high enough for fake signals to be reported as strong evidence for them^{3.4}.

The subjective point of view is expressed in a provocative way by de Finetti's[11]

``PROBABILITY DOES NOT EXIST''.