Understanding the works of intellectuals
These authors wrote for a particular audience and you, as a person living in 2017, are not who they had in mind. Part of comprehending text is being able to attach it to something that provides context to the text. You probably don't have this. Before reading books such as these try reading a modern summary that also provides context. This will give you a better chance at comprehension.

Additionally, the style of writing has evolved substantially since these books were written. Today authors (should) write for clarity. This wasn't always the case.