Full Idea
The probability that two events will both happen is the probability of the first [multiplied by] the probability of the second on the supposition that the first happens.
Gist of Idea
The probability of two events is the first probability times the second probability assuming the first
Source
Thomas Bayes (Essay on a Problem in the Doctrine of Chances [1763]), quoted by Dorothy Edgington - Conditionals (Stanf) 3.1
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.10