ABSTRACT
Probabilistic concepts, directly or indirectly, pervade many aspects of human
activities, from everyday situations to more advanced and specific applica-
tions in natural sciences, engineering, economy and politics. It is the scope
of this introductory chapter to discuss the fundamental idea of probability
which, as we wi l l see, is not so obvious and straightforward as it may seem.
In fact - in order to deal with practical problems in a first stage and to arrive
at a sound mathematical theory later - this concept has evolved through
