If philosophy has a long history, it is necessary to distinguish the practice of philosophy from the simple study of past doctrines. Sometimes attenuated or even erased, this distinction is crucial. Many thinkers appeal to the earlier philosophies to support, inspire, or criticize them: there is an appeal to history and a common cultural background, but it does not make from philosophy a historical discipline. Philosophical practice is not merely a gloss on the philosophy of preceding epochs, so it must be distinguished from the history of philosophy.
The history of philosophy consists in attempting to reconstruct, to understand, to interpret, even to criticize, the positions and theses of thinkers such as Plato, Thomas Aquinas, Hegel, etc. It is less a matter of evaluating the philosophical relevance or the current interest of these philosophers than of knowing what they really said, and of restoring their thoughts in their contexts of appearance. This work of study also deals with philosophical currents (ancient skepticism, neo-nantism), or questions debated in history (the dualism of the soul and the body, the quarrel of the universals) belonging also to the history of philosophy.
Philosophy, taken as an activity, aims to study and answer questions related to a problem, an area or branch of philosophy. It goes without saying that this practice constantly leads to referring to the earlier philosophers, but the relation to history here is different from that which the historian of philosophy would have. In such a case, the philosopher does not aim at knowing what he has thought; he seeks to reinstate this thought in his personal argumentation; he is instrumentalizing the preceding philosophies in order to justify his thought and to reveal his own point of view. The essence of this practice is to answer problems, to ask questions, if necessary using the history of philosophy.