ABSTRACT
With the mathematical treatment of probability by Fermat and Pascal in the seventeenth century, to be followed by the Bernoullis and Laplace and others in the eighteenth, there originated another analysis of the word, which call the possibility view. The possibility view of probability was in origin more or less loosely connected with the additional demand that the possibilities in question should all be 'equal'. The chapter discusses the relation of axiomatic probability to the probability concepts of the frequency, the possibility and the psychological views. It outlines an axiomatic and formalized deductive system for the relation P and this system will be called the Calculus of Probability. With the aid of the general principle of logic which have called the Principle of Identity it is possible to prove that the probability-relation is extensional, meaning that not only identical but also co-extensive properties are substitutable for each other in the relation.
