The tradition of the particle concept goes back to traditional metaphysics
and ancient philosophy. The idea that matter is made up of microscopic
constituent parts stems from ancient atomism. At the very beginnings of
modern physics, it was taken up by Galileo, Descartes, and Newton. Newton
thought that there are atoms of matter and light, but with the methods of
Newtonian mechanics and optics they were beyond the reach of experiments.
In the 19th century, the wave theory of light succeeded and electrodynamics,
thermodynamics, statistical mechanics, and the kinetic theory of heat were
developed. Two hundred years after Newton’s Principia, the experimenters
discovered cathode rays, X rays, and radioactivity, and began to investigate
the atomic and subatomic structure of matter. At the same time, however,
the empiricist Ernst Mach still doubted the existence of the unobservable
microscopic constituent parts of matter and asked for more natural ideas
about the structure of matter.