That is because the works themselves contain such internal evidence.

There's internal evidence that he's currently a much more important man than he was a week ago.

Internal evidence is the actual details present inside a given work.

As to the notes, internal evidence for once is explicit.

Based on internal evidence, the collection has been dated to around 1970.

There is no internal evidence to locate these works either in time or place.

Characters go unidentified, and it is not always possible to identify them from internal evidence.

Internal evidence suggests that the work was composed in or around the 1480s.

Indeed, all internal evidence suggests that it was probably the author's intention.

But nothing can be said with certainty, for want of both external and internal evidence.