 There are other formalizations that give up normalization; that give up countable additivity, and even additivity; that allow probabilities to take infinitesimal values (positive, but smaller than every positive real number); that allow probabilities to be imprecise — interval-valued, or more generally represented with sets of precise probability functions; and that treat probabilities comparatively rather than quantitatively. (See Fine 1974, Halpern 2003, Cozman 2016, Fine 2016, Hawthorne 2016, Lyon 2016.) For now, however, when we speak of ‘the probability calculus’, we will mean Kolmogorov’s approach, as is standard. See Hájek and Hitchcock (2016b) for a relatively non-technical introduction to it, intended for philosophers.