Probabilities are measures of how certain one is about something. Thus, for example if one is going to estimate how likely a coin is to come heads one will say .5. This doesn’t change the fact that the coin either will come up heads or come up tails. But I don’t know which one it will be. Similarly, if something in the past may or may not have occurred, then you can assign numeric estimates to how likely it is to have occurred.
Probabilities are measures of how certain one is about something. Thus, for example if one is going to estimate how likely a coin is to come heads one will say .5. This doesn’t change the fact that the coin either will come up heads or come up tails. But I don’t know which one it will be. Similarly, if something in the past may or may not have occurred, then you can assign numeric estimates to how likely it is to have occurred.