I'm curious about the nature of the evidence for three quarks -- two up and one down -- in the proton. I assume the data is derived from high energy scattering experiments, and looking at the angles and momenta of the scattered particles. But I don't see how, from such experiments, one can say there are two ups and one down, rather than their simply existing in a ratio of 2:1. In other words, might there be six quarks rather than three (four up, two down)? Is the determination of three based more on theoretical arguments, rather than experimental data?