Rule 2 for history of philosophy: respect the text
Rule 2: Respect the text
This is my version of what is sometimes called the "principle of charity." A minimal version of this rule is that we should assume, in the absence of fairly strong reasons for doubt, that the philosophical texts we are reading make sense. This holds not only for outstanding famous thinkers but also for lesser lights: even if they were not earth-shattering innovators, they usually didn't just write rubbish. Here it's also worth bearing in mind that until very recently (the last century) any text that has survived to get into your hands has already been through a process of selection by earlier readers. So they are likely to be reasonably good. But even without this observation, it still seems obvious (to me at least) that useful history of philosophy doesn't involve looking for inconsistencies and mistakes, but rather trying one's best to get a coherent and interesting line of argument out of the text. This is, of course, not to say that historical figures never contradicted themselves, made errors, and the like, but our interpretations should seek to avoid imputing such slips to them unless we have tried hard and failed to find a way of resolving the apparent slip.