Thursday, October 25, 2018

Sibling rivalry and the beginning of time

Greg Koukl has this tactic he calls "sibling rivalry." That's when somebody raises two objections, often unrelated to each other, but these objections are inconsistent with each other. The tactic is just to point it out. You say something like, "When you were objecting to this thing, you said P, but now that you're objecting to this other thing, you say not-P. Which is it?"

I wanted to mention one example of this I was just thinking about today. There are two philosophical arguments for why time had to have had a beginning. There are more than two, of course, but there's just two I want to talk about right now. One of them is the argument from the impossibility of an actual infinite. The other is from the Grim Reaper Paradox.

In response to the argument from the impossibility of an actual infinite, people will sometimes point out that a line of any finite length can be divided infinitely, or they point out that a line of any finite length is made up of an infinite number of points.

In response to the Grim Reaper Paradox, people will point out that once you reach the Planck time, you cannot divide time any further, so you cannot fit an infinite number of Grim Reapers into a finite interval of time.

These objections are at odds with each other. If Planck time was all that prevented us from carrying out a thought experiment involving Grim Reapers, then surely Planck length would just as well prevent us from carrying out a thought experiment involving the division of some length. But if Planck length is irrelevant to the thought experiment involving the division of some finite length, then Planck time is irrelevant to the thought experiment involving Grim Reapers.

So one of these objections fails no matter how you look at it. That means at least one of these arguments for the beginning of time survives the objections.

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