Centuries ago, Greek philosophers (such as Democritus) postulated that atoms were the fundamental building blocks of matter. Then, in the 1900s Rutherford along with others discovered that atoms consisted of electrons surrounding a nucleus made up of protons and neutrons. With the technological improvement of particle accelerators, a plethora of new "fundamental" particles were discovered in the 1960s and the years that followed. I can't help but wonder; how do we know that particles such as quarks and leptons are not in turn, made up of even smaller particles? Perhaps our accelerators have not reached energies capable of showing us such evidence. One should note, that my question originally arose because I was reading an article about the ILC, and it made a comparison between leptons and hadrons (in reference to the LHC). It claimed that producing collisions with leptons at weaker energies than the LHC could produce, would be ideal for research because leptons are fundamental particles whereas hadrons are not and could be subject to colliding at various angles (which in turn, could alter the results).