 Error and uncertainty have two possible sources according to Bolzano: either the premises that one presupposes are uncertain (or even false), or one has used a mere probability inference (WL III, 265 f.), i.e., an inference whose conclusion asserts the truth of a proposition \(s\) itself instead of merely asserting the truth of the proposition [\(s\) is probable] (WL I, 182 ff., WL II, 510, WL III, 212 f.). Considerations of this kind amount to quite a refined epistemological system of subjective probability, in which Bolzano distinguishes degrees of credibility and of assurance by using his probability theory (WL III, 274–288). Unfortunately, these investigations are little known, since they are only developed in the third volume of his Theory of Science which receives far less attention than it deserves.