[francoi8]

I think his argument is wrong and the fallacy comes when making the hypothesis the is a "50 percent chance that it was somewhere in the middle portion of the wall's timeline". It seems like a relatively reasonable assumption in that context but it is actually very arbitrary.

Imagine applying his reasoning to any building that has just been completed. Say we visit it the following day, make the 50% hypothesis and then conclude the building has a 50% chance of falling within 8 hours to 3 days. Obviously the conclusion is wrong as vastly more than 50% of buildings last longer than 3 days. We see in this case that postulating that there is a 50% chance that we visited the building somewhere in the middle portion of the building's timeline was completely unreasonable - we know from experience that buildings tend to last years.

So basically his estimation of when humans go extinct derives from a guess that may or may not be correct and in any case seems very arbitrary.

[skykooler]

Another way of looking at it: instead of imagining you're at a random time in humanity, imagine you're a random human. The number of humans on earth has been increasing exponentially, so if you pick one at random they are most likely going to be from the last two centuries. This skews the results by making it more likely that the prediction is being made at this particular point in time (or a bit later if the population continues growing), which means that it is a non-uniform sample.

[tgb]

If all the visitors to that building applied this logic, then 50% of them would correctly believe that they were among the middle 50% of visitors to the building. In that sense it is correct reasoning without making any assumptions.