truth rather than tentative hypothesis. And all of them differed from Mach in believing that natural science as it is is a specimen of knowledge. They did not seek a critique and restructuring of natural science à la Mach, but a logical analysis exhibiting the connection of the whole scientific superstructure with its observational base. They failed, and the tale thereof has oft been told.4 But confronted with their failure they did not follow through on their empiricist conviction and toss out that