quote from Schopenhauer’s essay, ‘How to think for oneself’, §268:“the most beautiful thought, if not written down, is in danger of being irretrievably forgotten.”It’s from the passage where he observes that Lichtenberg thought for himself in both senses of the phrase, unlike Herder.The original essay, “Selbstdenken” was part of Schopenhauer’s book Parerga und Paralipomena II. Last authorised edition, Erstausgabe Berlin, A. W. Hayn 1851, online textLooks like Povarnin was a Schopenhauer fan!

'картотек' (card index), to find the relevant section (it's section 51. The fascination with index cards, p.68). This is pretty much the only part of the booklet that discusses index cards, though.

S. I. Povarnin, How To Read BOOKS (1924)