The reader will observe that the Law of Causation does not state (as some modern writers most unfairly would have us believe) that Everything that exists has a cause. In this form it is quite untrue, since God is uncreated and uncaused. If it were worded thus, the objection, that we first formulate our universal law and then exclude from it Him on Whom all existence depends, would be perfectly valid. But this is entirely to misrepresent our position. It is one of the unworthy devices of the enemies of a priori philosophy.
Fr. Richard F. Clarke, Logic, 78-79 (H/T Edward Feser)