Rule 9: Learn the terminology
Another obvious one, perhaps, but also worth mentioning. Not all philosophers develop their own technical or semi-technical vocabulary, but many do. (Sometimes even those who officially make a big deal out of not worrying about terminology, like Plato.) When reading any philosopher, you need to know which words have a technical meaning and what they mean – this obviously requires knowing at least enough of the primary language to track the terms in question. (I actually considered having a more general rule to the effect of “learn the primary language,” but I worry that this could be discouraging: please do read Plato, even if you can’t read Greek! Still, it really does go without saying that there is a significant sense in which you can’t in fact read Plato if you can’t read Greek.)
This is another rule that has to do with avoiding anachronism. The more we know about a philosopher’s language, including not only the way terms were generally used at his or her time but also the way that this philosopher in particular uses terms, the less likely we are to import our own assumptions about what these terms must mean. There are many examples where scholars have pointed out that interpreters have mistakenly been taking a given word to mean what we now today would mean by it, whereas actually it meant something different – one that comes to mind is “cause” in Aristotle. The best way to guard against such mistakes is to track the use of a word across the philosopher’s works, using context (both in these works and in other works of the time) to get a better grip on exactly what the word means.
I had the hardest time
I had the hardest time figuring out Kant's terminology in The Critique of Pure Reason. I found that in the end, I simply had to ask the professor! Now I'm feeling like a cheater...
Well, I vaguely remember Kant himself giving a verbal spanking to anyone who used the word 'idea' in a broad sense. I presume he was thinking of his contemporary German audience and people who use the word casually to mean any thought whatsoever. So maybe I won't feel so bad if his contemporaries were as loose with language as I am.