This rule having been confirmed, I shall now proceed to prove that there is in being a being who has no reason for its being. Such a being is either contingent or necessary. If it is necessary, then the point we sought to prove is established. If on the other hand it is contingent, that which is contingent cannot enter upon being except for some reason which sways the scales in favor of its being and its not-being. Therefore contingent beings end in a Necessary Being.
Contingent things have a cause outside themselves.
A cause must itself be either necessary or contingent. If that cause is necessary, then there is a necessary being. If contingent, then it must have a cause, which will also be either necessary or contingent. This will lead to an infinite regress. If there is an infinite regress, then nothing can exist.