The distinguished historian of philosophy, Peter Adamson (from "without any gaps" fame; the podcasts are a big hit), has articulated "some guidelines encapsulating what I see as good practice in studying the history of philosophy" (quoted from rule 1) on his blog. He is up to the ninth rule as I write this. The rules themselves are an "open-ended encouragement to strive for an ideal which is not practically reachable" (see rule 4; see also rule 8). I very much like this feature of the rules because it encourages us to think of our practice as an open-ended process (and puts the idea of a gapless history in nice perspective). There is a further worthy feature of the rules that is never stated explicitly, but a kind of animating principle behind them: to treat the authors we are studying as serious thinkers in their own right--a judgment that vindicates, in part, earlier readers; for, "any text that has survived to get into your hands has already been through a process of selection by earlier readers," (rule 2).
Before I get to a more principled qualm about his rules, I want to offer one immanent criticism of the first rule: "It's possible for the same idea to appear independently more than once." Yes, this is possible and undoubtedly occurs, and it might generate interesting questions about the conceptual possibility space available to certain philosophers such that 'forced moves' get rediscovered--I call such forced moves: social-conceptual-necessitation. (Having said that, I thought his example of Al-Ghazali and Hume on causation especially poorly chosen; (a) we know that Malebranche was almost certainly familiar with Al-Ghazali through discussions in Aquinas and Suarez [I apologize for forgetting where I first read this!], and Hume certainly knew his Malebranche; (b) until Dario Perinetti has completed his study of the La Fleche library we don't really know what Hume (and Descartes) might have read in his most formative period.) But while I am no fan of a certain kind of lineage-tracing popular among historians of ideas, this rule also has the likely tendency to discourage inquisitiveness. In particular it is very tempting to assume that 'great' thinkers didn't read or read little--some philosophers (Descartes, Spinoza, Carnap, Lewis, etc.) even play with a recurring Meme along these lines.* But we need/ought not trust their self-presentation in these matters. We miss moves in an author if we are unaware of her interlocutors. My concern is not just a matter of getting the context right (cf. rule 4); we literally do not notice if some options/alternatives are quietly foreclosed. It matters, for example, that Spinoza had More's Utopia in his library (and almost certainly read Bacon's New Atlantis); this means he was almost certainly aware of alternative ways to think about, say, gender relations and ways to organize religious-theological pluralism in society (as well as property relations, Biblical hermeneutics, etc.) even if he never went back to Plato. Aware of this, we might upon re-reading Spinoza notice that he has developed resources to reject those alternatives or we might start to see cracks in his edifice (etc.). [Below I return to rule 1!]
Even so, the rules are problematic, especially because they come with the weighty authority of an emiment and popular scholar. Leaving aside the fact that Adamson seems to be ignorant of the methodological discussions among other historians of philosophy of the last few years, Adamson never recognizes that there are alternative ways to study the history of philosophy in which some of the vices he recognizes may well be necessary virtues. I'll pick one example: Adamson is extremely concerned to have the historian avoid anachronism (see rule 4, 7, 8, 9). As it happens I have more tolerance for the kind of un-selfconscious anachronism that trained historians of philosophy are taught to reject; such un-selfconscious anachronistic readings of a text can, but need not, be interesting on their own terms ('what axioms in S4 or S5 would Spinoza or Leibniz reject?' can be hugely illuminating both to our understanding of Spinoza or a time-slice of Leibniz and to the modern student of modal logic). But that's not what I have in mind in defending here.