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Collect: O God who upholds all things in being by your proper causality, grant that our minds and ascend unto thee through the things thou hast made, through Christ our Lord. Amen.
In the Sed Contra, St. Thomas proves this thesis from authority. While this certainly does not matter to non-Catholic agnostics, it is de fide (of faith) that God can be demonstrated to exist from reason alone. This is taught at Vatican I, "The same Holy Mother Church holds and teaches that God, the beginning and end of all things, can be known with certitude by the natural light of human reason from created things."
In the Respondeo, St. Thomas begins by distinguishing the two ways in which we may prove something to exist. The first is a priori, i.e., from the proper cause of its existence. Cardinal Mercier explains, "An a priori argument is one in which the middle term [the term used in both the major and minor premise] is prior in reality to the predicate of the conclusion, one which proceeds from the cause or reason to effect or result. In an a posteriori argument the progress is just the reverse, being from effect to cause."