I'm a bit confused. I've got about 40 something references for my dissertation, and I've got to split them up into primary and secondary resources. I sort of know the difference between primary and secondary, primary happening at the time, etc, secondary interprets.
But what about books like J.S. Mill's on liberty? Or such like that; theoretical stuff like that, where they sort of interpret old ideas and concepts, but add new interpretations and such, i'm confused.