Scientist: Given the human tendency to explore and colonize new areas, some people believe that the galaxy will eventually be colonized by trillions of humans. If so, the vast majority of humans ever to live would be alive during this period of colonization. Since all of us are humans and we have no reason to think we are unrepresentative, the odds are overwhelming that we would be alive during this period, too. But, because we are not alive during this period, the odds are slim that such colonization will ever happen.
The scientist's argument proceeds by taking for granted that dependable predictions about the future cannot ever be made simply on the basis of the present facts.