Full Idea
Bayesianism says ideally rational people should have degrees of belief (not all-or-nothing beliefs), corresponding with probability theory. Probability of H, given evidence E, is prob(H) X prob(E given H) / prob(E).
Gist of Idea
Probability of H, given evidence E, is prob(H) x prob(E given H) / prob(E)
Source
Paul Horwich (Bayesianism [1992], p.41)
Book Reference
'A Companion to Epistemology', ed/tr. Dancy,J. /Sosa,E. [Blackwell 1993], p.41