Probability theory is the body of knowledge that enables us to reason formally about uncertain events. The populist view of probability is the so-called frequentist approach whereby the probability P of an uncertain event A, written P(A), is defined by the frequency of that event based on previous observations. For example, in the UK 50.9% of all babies born are girls; suppose then that we are interested in the event A: 'a randomly selected baby is a girl'. According to the frequentist approach P(A)=0.509.