"The intuition behind the theory is that one atom is indicative of solidity and this is the heaviest; the second atom makes the liquid state possible and therefore it should be lighter; the third atom represents decay and it has (probably) no mass (this is not expressly stated); and the fourth is the constituent of light and heat and it definitely has no mass. It is astonishing that these atoms have turned out to be similar to proton, electron, neutrino, and photon."