Any serious study of philosophy will involve a mixture of historical and topic-based study, since if we don’t know about the arguments and errors of earlier philosophers, we cannot hope to make a substantial contribution to the subject. Without some knowledge of history philosophers would never progress: they would keep making the same mistakes, unaware that they had been made before. And many philosophers develop their own theories by seeing what is wrong with the work of earlier philosophers.
Nigel Warburton, Philosophy: The Basics (Fifth ed., 2013), Introduction