Beliefs on matter
Aristotle believed that matter was continuous, so that one could divide a piece of matter into smaller and smaller bits without limit.
Democritus thought that matter was grainy, and that everything was made up of large numbers of various different kinds of atoms (which means indivisible in greek)
Einstein settled this argument by providing a paper on Brownian motion where he explained as the effect of atoms of the liquid colliding with the dust particles.
Already years before J.J. Thomson demonstrated the existence of a particle of matter, the electron, that had mass less than one 1000th of that of the lightest atom.