A Treatise on Probability was published by John Maynard Keynes while at Cambridge University in 1921. The Treatise attacked the classical theory of probability and proposed a "logical-relationist" theory instead. In a 1922 review, Bertrand Russell, the co-author of Principia Mathematica, called it "undoubtedly the most important work on probability that has appeared for a very long time," and a "book as a whole is one which it is impossible to praise too highly." Keynes saw numerical probabilities as special cases of probability, which did not have to be quantifiable or even comparable.