The philosophically significant aspects of a historical theory can sometimes be illuminated by comparing them with later views. This does not involve anachronism, provided that one does not maintain that past philosophers said something that they did not say or mean. Reading older philosophical works as philosophical involves understanding them as particular answers to questions which are dealt with in other ways by other thinkers. This systematic aspect is lacking in non-philosophical doxographic history of philosophy, which itself is an important branch of research