It is now tradition in literature to associate the probabilistic theory to the spread of gambling. Both probability theories and gambling can be seen as one of the consequences of the secularization process that began in the middle of the seventeenth century with the end of the religious wars and invested different areas of private and public life. While statistical and mathematical aspects were formalized in the 1930s, there has not been a systemization at a conceptual level. The issue of how to interpret probability remains open. This essay focuses on the main schools of thought (classic, frequentist, subjectivist and logistic) taking into consideration the historic context in which they developed and the instances they represent. Particular attention is given to subjectivism and to B. de Finetti’s work. The last paragraph is dedicated to the Bayesian Theory which represents a way to think of inference, rather than a formula to calculate probability. The essay concludes with two applications of Bayes’ Theorem to electoral data of 2008.