No more useful inquiry can be proposed than that which seeks to determine the nature and the scope of human knowledge.... This investigation should be undertaken once at least in his life by anyone who has the slightest regard for truth, since in pursuing it the true instruments of knowledge and the whole method of inquiry come to light. But nothing seems to me more futile than the conduct of those who boldly dispute about the secrets of nature... without yet having ever asked even whether human reason is adequate to the solution of these problems.

Rules for the Direction of the Mind in Key Philosophical Writings (1997), pp. 29-30