(I) Neutrinos have been eluding physicists ever since Pauli first hypothesized their existence in 19(II) In the physical universe, what goes in, equals what comes out, in one form or another. (Ill) So there will be no need to postulate a particle that cannot be detected. (IV) But physicists had noticed that when radioactive atoms spat out electrons and transformed into other kinds of atoms, some of the original energy appeared to be missing. (V) Pauli proposed that it had been carried away by a virtually invisible particle.