This paper argues that probability is not an objective phenomenon that can be identified with either the configurational properties of sequences, or the dynamic properties of sources that generate sequences. Instead, it is proposed that probability is a function of subjective as well as objective conditions. This is explained by formulating a notion of probability that is a modification of Laplace's classical enunciation. This definition is then used to explain why probability is strongly associated with disordered sequences, and is also used to throw light on a number of problems in probability theory.