From Sparks Notes comes this succinct paragraph.
I take this to mean that Science must end in the brute fact that there is our universe (or more?) just simply is and cannot be explained or known to Science or else it ends in an infinite regress since each cause was itself an effect of a previous cause, which was itself the effect of a prior cause, and so on forever. Either situation is a far cry from the certainty Descartes sought.In fact, in order to be a proper Cartesian rationalist (i.e. someone who believes that the entire world can be explained in terms of a chain of logical connections and that we have access to this explanation) you have to believe in the possibility of an ontological argument. Without an ontological argument, explanation must either end in some brute, unexplained fact, or turn into an infinite regress, where the there is no end to explanation. In order to ensure that explanation comes to a final halt (and a halt with no loose, unexplained end), it is necessary that there be some level of reality that causes itself, something that is its own explanation. The only plausible candidate for an entity that is its own explanation is God. And the only way for God to be his own explanation is for some version of the ontological argument to work.