Not sure I can get behind this. We read primary sources in history, but in those cases it is to get understanding that can only come from a first person account

Not sure I can get behind this. We read primary sources in history, but in those cases it is to get understanding that can only come from a first person account, like subjective experience. With philosophy, it is argued that Aristotle's thoughts can only be fully conveyed BY Aristotle (aside from the hero worship of some philosophers I know). My sense is that you're really on to something with empiricism. Philosophy is now largely about purely social phenomena, like epistemology and ethics, that don't have a physical "correct" answer (at least to most people--maybe not Kant). That being the case, there really is something unique about, for example, Aristotle's take on virtue ethics, since there is no virtue ethics outside of his conception of it (and our practicing of it). So the pure thing being studied in philosophy is Aristotle's conception and the surrounding literature of virtue ethics, whereas with physics and Newton his theories and equations are pure things and there is no value to his particular exposition.