What does 'Study the past if you would define the future' mean?

First of all, the quote has a popular mistake. The actual quote uses to the "divine" not "define." What does divine mean? Archaically, it meant the ability to prophesy (i.e., to predict the future).

In plain English, the quote says, "If you want to get a good idea of what the future is going to be, then you should study your past."

Using reverse logic, the negative-inverse is that if you do not study your past, you will not know the likely future. The inference taken from this in the study of philosophy is that you are more likely to make the same mistakes if you do not study your past.

As the prior post stated, "If you know the past (cause) then you can guess the future (effect)."