Now, if we consider closely the use of historical proofs, four points of view readily present themselves for the purpose.
First, they may be used merely as an explanation of an idea. In every abstract consideration it is very easy to be misunderstood, or not to be intelligible at all: when an author is afraid of this, an exemplification from history serves to throw the light which is wanted on his idea, and to ensure his being intelligible to his reader.
Secondly, it may serve as an application of an idea, because by means of an example there is an opportunity of showing the action of those minor circumstances which cannot all be comprehended and explained in any general expression of an idea; for in that consists, indeed, the difference between theory and experience. Both these cases belong to examples properly speaking, the two following belong to historical proofs.
Thirdly, a historical fact may be referred to particularly, in order to support what one has advanced. This is in all cases sufficient, if we have only to prove the possibility of a fact or effect.
Lastly, in the fourth place, from the circumstantial detail of a historical event, and by collecting together several of them, we may deduce some theory, which therefore has its true proof in this testimony itself.
Clausewitz, On War, Book II, Chapter VI. He goes on to consider how much of a role historical authenticity plays in each of these uses and the ways in which things can go wrong. This is one of the nice things about Clausewitz's philosophical work on war: he is very thoughtful about methodology, and he deserves more attention on that note if no other. I've already noted his comments on hypothetical scenarios.
