In the second paragraph of Introduction to his Prolegomena to Any Future Metaphysics, Immanuel Kant says that scholars should not confuse philosophy with the history of philosophy.
Here’s the excerpt:
“There are scholarly men, to whom the history of philosophy (both ancient and modern) is philosophy itself; for these the present Prolegomena are not written. They must wait till those who endeavor to draw from the fountain of reason itself have completed their work; it will then be the historian's turn to inform the world of what has been done. Unfortunately, nothing can be said, which in their opinion has not been said before, and truly the same prophecy applies to all future time; for since the human reason has for many centuries speculated upon innumerable objects in various ways, it is hardly to be expected that we should not be able to discover analogies for every new idea among the old sayings of past ages.”
(Source: Introduction to his Prolegomena to Any Future Metaphysics by Immanuel Kant)