"the shadow of lost knowledge"

"A certain amount of knowledge you can indeed with average faculties acquire so as to retain; nor need you regret the hours you spent on much that is forgotten, for the shadow of lost knowledge at least protects you from many illusions.
     "But you go to a great school, not for knowledge so much as for arts and habits; for the habit of attention, for the art of expression, for the art of assuming at a moment’s notice a new intellectual posture, for the art of entering quickly into another person’s thoughts, for the habit of submitting to censure and refutation, for the art of indicating assent or dissent in graduated terms, for the habit of regarding minute points of accuracy, for the art of working out what is possible in a given time, for taste, for discrimination, for mental courage and mental soberness."

     William Johnson Cory, Eton reform II (London:  Longman, Green, Longman, and Roberts, 1861), 6-7.  I was put onto this by Joseph Epstein, "How to re-read," The lamp:  a Catholic journal of literature, science, the fine arts, etc. no. 19 (Christ the King 2023):  41 (where the author, but not the source, is given, and which I have corrected against the original).