Aquinas, Hume, and the Cosmological Argument I will attempt to address your second question in how to respond to Hume. Recently I was introduced to something call the Grim Reaper Paradox and when altered, shows that a world with an infinite regress cannot exist.
The original paradox goes something like this:
There are an infinite number of Grim Reapers that each have a deadline at which he is scheduled to kill person X. If X is alive at a time that a Grim Reaper is scheduled to kill X, he will kill X. If X is not alive, the Grim Reaper does nothing. Grim Reaper 1 is scheduled to kill X at 9:00AM, Grim Reaper 2 at 8:30AM, Grim Reaper 3 at 8:15AM, and so on continuously being scheduled at half the minute hand of the previous Grim Reaper for an infinite number of Grim Reapers. If you were to pick any of the Grim Reapers, X would not be alive when it becomes the time at which that Grim Reaper is supposed to kill X. Meaning no Grim Reaper would kill X because there will always be a Grim Reaper scheduled to kill X before their time comes.
The modified version of the paradox:
There are still an infinite number of Grim Reapers, but this time instead of killing anyone, they are assigned to a year and are tasked to pass along a note to the next Grim Reaper. Grim Reaper 1 is assigned to 1 B.C. Grim Reaper 2 at 2 B.C. and so on. This is also assuming that the past is indeed infinite. Each Grim Reaper receives a piece of paper on the first day of every year from his predecessor and hands it to his successor on the last day of the year he is assigned. If the paper is blank when received, he will write the number of the year he assigned on the piece of paper and pass it on. If the paper already has a piece of paper on it, he will just pass it along without writing anything on it. The paradox is, there must be a number on the piece of paper by the time it reaches the year 1 A.D. But there is no number that can be on that paper because the past would be infinite and no Grim Reaper would ever get a note without a number and there would be no Grim Reaper that would write down their number.
This leads to the argument that:
1. If there could be an infinite regress of causes, then the scenarios provided above would be possible.
2. But these scenarios are impossible.
3. So, there could not be an infinite regress of causes.