[cafard]

Donald Knuth wrote of Robert Floyd that

  (Bob used to say that he was planning to get a PhD by the “green stamp method,” namely by saving envelopes addressed to him as “Dr. Floyd.” After collecting 500 such letters, he mused, a university somewhere in Arizona would probably grant him a degree.)

See https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1299661

I suppose that Stallman must have received enough letters.