mathematics used, was just assumed to hold. Quantum mechanics, with its incompatible measurements, was born rather by chance in an atmosphere of great positivistic zeal, in which only the obviously measurable had scientific respectability. Speculation about occult "unobservable" quantities was viewed as vacuous metaphysics, which should surely form no part of a mature scientific attitude. Soon the "unmeasurable, " once only disreputable, vanished altogether. One had first been told not to worry about it; then, as dogma got more carefully defined, one was assured that the unobserved was just not there. This made it easier not to think about it and to avoid hazardous metaphysical temptation.