 Beyond the usage of the method of idea-variation for his truly pioneering definitions of logical truth and logical consequence and related concepts (such as the concepts of satisfiability and compatibility), Bolzano made use of this method also for a series of other purposes, above all in his development of the theory of probability (WL II, 77–82, 171–191, 509–514, WL III, 136–138, 263–288, 559–568, RW II, 39–49, 57–61, 66–71). Bolzano’s theory of probability is based on his distinction of different degrees of validity of a proposition \(s\) with respect to a sequence \(i\) of ideas. This degree of validity of s with respect to i is representable as a fractional number \(\bfrac{m}{n}\) where \(n\) is the number of all possible variants of \(s\) with respect to \(i\) and \(m\) is the number of true variants of \(s\) with respect to \(i\). If all variants of \(s\) with respect to \(i\) with non-empty subject ideas are true, \(m = n\) and \(\bfrac{m}{n} = 1\), i.e., \(s\) is universally valid with respect to \(i\); if all variants are false, \(m = 0\) and \(\bfrac{m}{n} = 0\), i.e., \(s\) is universally contravalid with respect to \(i\) (WL II, 81 f.). The logical degree of validity of s is thus the degree of validity of \(s\) with respect to \(i^s\), i.e., with respect to a certain sequence of all simple extra-logical ideas contained in \(s\). 