Bertrand Russell writes:
There is no logical impossibility in the hypothesis that the world sprang into existence five minutes ago, exactly as it then was, with a population that "remembered" a wholly unreal past. There is no logically necessary connection between events at different times; therefore nothing that is happening now or will happen in the future can disprove the hypothesis that the world began five minutes ago.... I am not here suggesting that the non-existence of the past should be entertained as a serious hypothesis. Like all sceptical hypotheses, it is logically tenable but uninteresting (The Analysis of Mind, 1921, p. 159-160).Wait... what?! We can't prove that the world has existed for more than five minutes, but who cares, bo-ring?
I'd think rather the opposite: We can prove that the world has existed for more than five minutes. And if I didn't think I could prove that fact, I wouldn't say ho-hum, whatevs, maybe everything I thought really happened is just an illusion, *yawn*.
Okay, well, "prove" is too strong. Logical certainty of the 2+2=4 variety, I can't provide. But I think I can provide, at least to myself, some good empirical evidence that the past existed five minutes ago. I can begin by opening up the New York Times. It tells me that Larry Kwong was the Jeremy Lin of his day. Therefore, the past exists.
Now hold on, you'll say. I've begged the question against Russell. For Russell is asking us to consider the possibility that the world was created five minutes ago with everything exactly as it then was, including the New York Times website. I can't disconfirm that hypothesis by going to the New York Times website! The young-world hypothesis and the old-world hypothesis make exactly the same prediction about what I will find, and therefore the evidence does not distinguish between them.
Here I need to think carefully about the form of the young-world hypothesis and the reasons I might entertain it. For example, I should consider what reason I have to prefer the hypothesis that a planet-sized external world was created five minutes ago vs. the hypothesis that just I and my immediate environment were created five minutes ago. Is there reason to regard the first hypothesis as more likely than the second hypothesis? I think not. And similarly for temporal extent: There's nothing privileged about 5 minutes ago vs. 7.22 minutes ago vs. 27 hours ago, vs. 10 seconds ago, etc. (perhaps up to the bound of the "specious present").
So if I'm in a skeptical enough mood to take young-world hypotheses seriously, I should have a fairly broad range of young-world hypotheses in mind as live options. Right?
In the spirit of skeptical indifference, let's say that I go ahead and assign a 50% probability to a standard non-skeptical old-world hypothesis and a 50% probability to a young-world hypothesis based on my subjective experience of a seeming-external-world right now. Conditional on the young-world hypothesis, I then further distribute my probabilities, say, 50-50% between the I-and-my-immediate-environment hypothesis and the whole-planet hypothesis. The exact probabilities don't matter, and clearly there will be many other possibilities; this is just to show the form of the argument. So, right now, my probabilities are 50% old-world, 25% young-small-world, and 25% young-large-world.
Now, if the world is young and small, then all should be void, or chaos, beyond the walls of this room. (In a certain sense such a world might be large, but the Earthly stuff in my vicinity is small.) In particular, the New York Times web servers should not exist. So if I try to visit the New York Times' webpage, based on my entirely false seeming-memories of how the world works, New York Times content which is not currently on my computer should not then appear on my computer. After all the hypothesis is that the world was created five minutes ago as it then was, and my computer did not have the newest NYT content at that time. But, I check the site and new content appears! Alternatively, I open my office door and behold, there's a hallway! So now, I reduce the probability of the young-small-world hypothesis to zero. (Zero makes the case simple, but the argument doesn't depend on that.) Distributing that probability back onto the remaining two hypotheses, the old-world hypothesis is now 67% and young-large-world is 33%. Thus, by looking at the New York Times website, I've given myself empirical evidence that the world is more than five minutes old.
Admittedly, I'd hope for better than 67% probability that the world is old, so the result is a little disappointing. If I could justify the proposition that if the world is young, then it is probably small enough not to contain the New York Times web servers, then I can get final probabilities higher. And maybe I can justify that proposition. Maybe the likeliest young-world scenario is a spontaneous congealment scenario (i.e., a Boltzmann brain scenario) or a scenario in which this world is a small-scale computer simulation (see my earlier posts on Bostrom's simulation argument). For example, if the initial probabilities are 50% old, 45% young-small, 5% young-large, then after ruling out young-small, the old-world hypothesis rises above 90% -- an almost-publishable p value! And maybe the probability of the old-world hypothesis continues to rise as I find that the world continues to exist in a stable configuration, since I might reasonably think that if the world is only 5 minutes old or 12 minutes old, it stands a fair chance of collapsing soon -- a possibility that seems less likely on the standard non-skeptical old-world hypothesis. (However, there is the worry that after a minute or so, I will need to restart my anti-skeptical argument again due to the possibility that I only falsely remember having checked the New York Times site. Such are the bitters of trying to refute skepticism.)
You might object that the most likely young-small-world hypothesis is one on which there is an outside agent who will feed me the seeming-New York Times website on demand, purposely to trick me. But I'm not sure that's so. And in any case, with that move we're starting to drift toward something like Cartesian demon doubt, which deserves separate treatment.
[Slightly edited 6:33 pm in light of helpful clarificatory questions from Jonathan Weisberg.]