For me to explain this subject would require writing a long post. Since I do not want to make it tedious, I will explain it gradually.
Here first I give you a few points, which we need to first agree on them, so I can go on farther to give a more specific answer.
The first point is, the truth of the events of the past does not always require that they are written in a history book, because we can know something has happened based on the signs we find later. For example, consider the Bing Bang. When it happened no body was there to note it down, and we can never find a history book that describes that such an event happened, but through discovery of signs, and logical deductions we can know it happened. I call this 'point A', because I would refer to it later on in my future post.
Point B: Now, to describe another important point, I give an example as follows:
Suppose 'you' tell me that this Friday at 1 pm, in the city of so and so, and the street so and so, I will see a man, who is wearing a black hat, a green pants, a white shirt...he speaks German, his age is between 20 and 30. You keep giving 100 signs specific to him.
Then I tell you, No! Impossible! Trash! There are so many people who speak German. Just because i will find a man who speaks German does not mean it is the person you a talking about. Then You tell me: but i did not say any man who speaks German will be him, I said when you see a man who has ALL the 100 signs, then it is him. Because how can in the same place and the same time there appears more than one person having all these 100 signs?!! Virtually impossible. And if I see such a person in the same place and time, how can I still deny it?
Now, let's not please go any farther to see how these are applicable to what we are discussing. But just lets agree on these Two points if you find it fair.