E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A posteriori [Latin, from the latter].
An a posteriori argument is proving the cause from the effect. Thus, if we see a watch, we conclude there was a watchmaker. Robinson Crusoe inferred there was another human being on the desert island, because he saw a human footprint in the wet sand. It is thus the existence and character of Deity is inferred from his works. (See A PRIORI.)