The Bactra Review   Why So Slow?
See Popper's The Open Society and Its Enemies (London: Routledge, 1945, and numerous subsequent editions), esp. vol. I. Arguments very similar to Popper's were advanced some years earlier by L. Susan Stebbing in her Ideals and Illusions (London: Watts and Co., 1941), and since Stebbing knew Popper, and even helped him get a job, it's possible he was influenced by her on this point.