"Nobody has ever seen an electron. Nor a thought. You can't see a thought, you can't measure, weigh, nor taste it- but thoughts are the most real things in the Galaxy."

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Indeed, nothing more beautifully simplifying has ever happened in the history of science than the whole series of discoveries culminating about 1914 which finally brought practically universal acceptance to the theory that the material world contains but two fundamental entities, namely, positive and negative electrons, exactly alike in charge, but differing widely in mass, the positive electron-now usually called a proton-being 1850 times heavier than the negative, now usually called simply the electron.

— Robert Andrews Millikan