If I’m writing a history of military strategy I don’t have to be a successful military strategist myself because I can evaluate military strategies based on their results. I would give a lot of attention to Roman military strategies and not so much to those of the neighboring Italian states of Latium because the Romans won and the Latians lost. The task of studying the history of philosophy is harder: it’s as if I am given the strategy books of history’s generals but the knowledge of the success or failure of these strategies is barred to me because the world is hidden by a fog. I have no way of knowing whether the generals I’m studying won or lost. How do I figure out who to study? How do I figure out who to pay attention to? I need to evaluate Eisenhower without knowing if the D-Day invasion was a success or a fiasco.
If I want to evaluate whether Kant was an important or a minor figure I need to figure out for myself whether or not the central claims of the critique of pure reason make sense or they don’t. Otherwise I am just doing the history of publishing — there was a man in such and such a place who wrote some pages. I can’t even say who is influential or who is not, because the people who claim to be influenced by Kant may be incorrect.
The only way to do the history of philosophy is to do philosophy. Every historian is a good or bad philosopher.