A theorist today is hardly considered respectable if he or she has not introduced at least one new particle for which there is no experimental eviden… - Steven Weinberg

By the end of the nineteenth century the idea of the atom had become familiar... but not yet universally accepted. Partly because of the heritage of Newton and Dalton, there was a disposition to use atomic theories in England. ...Resistance to atomism persisted in Germany ...under the influence of an empiricist school... centered on Ernst Mach... many [German physicists and chemists] held back from incorporating into... theories anything that—like atoms—could not be observed directly. ...It is said that the opposition to Boltzmann's work by the followers of Mach contributed to Boltzmann's suicide...

There’s something I’ve been working on for more than a year — maybe it’s just an old man’s obsession, but I’m trying to find an approach to quantum mechanics that makes more sense than existing approaches. I’ve just finished editing the second edition of my book, Lectures on Quantum Mechanics, in which I think I strengthen the argument that none of the existing interpretations of quantum mechanics are entirely satisfactory.

Symmetry is not enough by itself. In electromagnetism, for example, if you write down all the symmetries we know, such as Lorentz invariance and gauge invariance, you don’t get a unique theory that predicts the magnetic moment of the electron. The only way to do that is to add the principle of renormalisability – which dictates a high degree of simplicity in the theory and excludes these additional terms that would have changed the magnetic moment of the electron from the value Schwinger calculated in 1948.