4 ms·

Not a blackboard, but just a slate. Paper was too expensive for him, so he used it only to write down results. Source: https://www.quantamagazine.org/three-puz

• cpach 6y ago
  That’s very fascinating. I guess it still is the case for many people in the world, that paper is too expensive. Whereas for many of us it’s readily available in massive, one might even say infinite, quantities.