The point of the thought experiment is to avoid using any priors. If you have a single data-point of an event E, taking time T to occur, then there are 2 possi

The point of the thought experiment is to avoid using any priors. If you have a single data-point of an event E, taking time T to occur, then there are 2 possibilities: A) Event E usually takes much much longer than T (>100T), and what you just witnessed is an anomaly, or B) Event E usually does take roughly time T to occur, and what you witnessed is within the range of what usually occurs. Both conclusions have an extremely high likelihood of being wrong, since the sample size is so small. But between the 2, conclusion B is more likely to be true than conclusion A.