Sheffer stroke the most important advance in logic? I think I once read, or heard, that Bertrand Russell once said that the discovery that all logical operators are expressible in terms of the Sheffer stroke was the most significant advance in logic since the publication of _Principia Mathematica_ , and that had he and Whitehead known about it beforehand, they would have proceeded completely differently. This is a really strange claim, so I imagine that I (or the person who told me) misunderstood, or misheard, or made it up completely. If Russell really did say something like this, what exactly did he say, and where and when?

Russell apparently made this comment in the 1925 edition of PM, where he introduced the (NAND) Sheffer stroke. See page 1 of The Evolution of Principia Mathematica. It says that Russell called it "the most definite improvement resulting from mathematical logic during the past fourteen years." But as the author notes, this is an odd comment, and the change is a technical triviality. I don't understand why Russell was so impressed with this either. I have never found it useful for proving anything, and I have never seen others use it either. It gets a passing mention at most.