At the heart of the debate between the Bayesian and frequentist worldview is the question of how we define probability.

In the frequentist worldview, probability is a notion that is derived from the frequencies of repeated events—for example, when we define the probability of getting heads when a fair coin is tossed as being equal to half. This is because when we repeatedly toss a fair coin, the number of heads divided by the total number of coin tosses approaches 0.5 when the number of coin tosses is sufficiently large.

The Bayesian worldview is different, and the notion of probability is that it is related to one's degree of belief in the event happening. Thus, for a Bayesian statistician, having a belief that the probability of a fair die turning up five is 1/6 relates to our belief in the chances of that event occurring.