1) We exist (or at least you do, Cogito, ergo sum, "I think, therefore I am") 2) Since we exist, something had to have lead to us 3) The thing that lead to us

1) We exist (or at least you do, Cogito, ergo sum, "I think, therefore I am") 2) Since we exist, something had to have lead to us 3) The thing that lead to us was either lead to in turn, or "just existed" 4) If you don't accept that something can "just exist", that leads to infinite regress (before A there was B, before B there was C... on to infinity) 5) It's not too presumptuous to prevent infinite regress by saying "the universe" is what "just existed". Since the possibility of the universe "just existing" was established by the previous reasoning, you can think if that's a reasonable stopping place when it comes to predictive power. Hawking suggested it was though the "projection" he used for making the argument was complex and and don't fully understand it. As far as I can tell, his argument was that the universe appearing to have a beginning was "merely" thermodynamic, and in the final quantum theory the beginning and the end would be closed, making the universe fully self contained. (Something about gravitational entropy fully canceling out all other entropy, making the universe sum to zero, with both the "beginning" and the "end" summing to the exact same value as each other (zero)).