 The main idea is that the premises of a valid argument can be uncertain, in which case (deductive) validity imposes no conditions on the (un)certainty of the conclusion. For example, the argument with premises ‘if it will rain tomorrow, I will get wet’ and ‘it will rain tomorrow’, and conclusion ‘I will get wet’ is valid, but if its second premise is uncertain, its conclusion will typically also be uncertain. Propositional probability logics represent such uncertainties as probabilities, and study how they ‘flow’ from the premises to the conclusion; in other words, they do not study truth preservation, but rather probability preservation. The following three subsections discuss systems that deal with increasingly more general versions of this issue.