by Mark R. Crovelli
Abstract: My claim that probability ought to be defined as a purely subjective measure of human belief has been challenged in a recent and interesting article on these pages by Arnold Baise (2011). Baise argues that probability ought to be defined, not as a purely subjective measure of human belief, as I have claimed, but rather in the following way:
Probability P(A|I) is a number between 0 and 1 that indicates how plausible it is that proposition A is true, based on information I. In addition, one could add that a probability of 1 indicates certainty that the proposition is true, while a probability of 0 indicates certainty that the proposition is false. (2011, p.3).
The reasoning that leads Baise to advance this definition for probability, however, is seriously and apodictically flawed. As a consequence, his definition for probability must be rejected as a viable alternative to my purely subjective definition.