When a coin is tossed, it does not necessarily fall heads or tails; it can roll away or stand on its edge. - William Feller

Historically, the original purpose of the theory of probability was to describe the exceedingly narrow domain of experience connected with games of chance, and the main effort was directed to the calculation of certain probabilities.

In stochastic processes the future is not uniquely determined, but we have at least probability relations enabling us to make predictions.

The notion of conditional probability is a basic tool of probability theory, and it is unfortunate that its great simplicity is somewhat obscured by a singularly clumsy terminology.