A few years ago at a party I was explaining some interpretive debate about Marx to a friend. Marx, I was saying, might have believed A, or he might have believed B, and there is evidence on both sides. My friend objected: what does it matter? Surely we should talk about whether A or B (or neither) is correct, not about which of them a guy from the Rhineland who died almost 150 years ago believed.
Prima facie, this is a pretty good objection. It is a good objection because a lot of people who study the history of ideas don’t seem to realize that it is an error to believe something merely because someone whom you consider to be wise believed it.
But I think the history of ideas is worth studying. Some of the reasons for this are pretty complicated (see here and here), but two are simple: