Note that a priori probabilities are set up before making the draws, on the basis of our prior knowledge of the situation, whereas a posteriori probabilities are set up after making the draws, on the basis of the results of the draws. [...] In our urn example, the conditions were the composition of the contents of the urn, and the actions were the making of a random draw. [...] One might say that the prob- ability of the success of a trial is a numerical measure of the likelihood that the trial will succeed, but this definition is circular. [...] In the case of the tossing of coins, the appear- ance of head or tail forms a set of two mutually-exclusive events. [...] The probability of the success or failure of a trial is said to be conditional if the success or failure is contingent in some way upon the outcome of some other (and usually) prior trial.