P1) Every event has a cause
P2) Something cannot cause itself
P3) A cause must preceed its effect
P4) There must have been a first event in the natural world
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C1) The cause of the first event in the natural world is supernatural in the sense that it is prior to (before) the natural world
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C2) That supernatural cause is god

(One way of cashing out one of Aquinas' 'five ways' to prove the existence of god)

This argument is regarded as a-posteriori.
'A-posteriori' means after experience.
The contrast is 'a-priori' which means before, or prior to experience.
An argument is a-posteriori if one or more of its premises relies on observations. It is thought that the premises rely on our experiencing causation and the like and so Aquinas offers 5 ways to prove the existence of god based on our experience of the natural world. Hume thought that knowledge of causation was a-priori rather than a-posteriori though so I guess he would think the argument from causation is really an a-priori argument...