What is this must? What function does it play?
if it be judged useful by those inquirers who desire an exact knowledge of the past as an aid to the interpretation of the future, which in the course of human things must resemble if it does not reflect it, I shall be content.
It's all Greek to me.
I think it means that if the future does not reflect the past (which I take to mean it is completely different from the past), then it must resemble the past. It is very unlikely the past and the future will be completely identical, so they must either be similar or completely different.
It's a very garbled way of saying it, if that's what it means.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
Your appreciation is welcome, James, but there is no need to write a new post to say thank you. Simply click the Like button on any posts you find helpful. It means that we don't have to open the thread again to read your new post and then find that it doesn't include any new information or an additional question.
Additionally, please note that a better title would have been What does "must" mean here?
Extract from the Posting Guidelines:
'Thread titles should include all or part of the word/phrase being discussed.'