CarlB called attention to this paper by Johan Hanssen: http://arxiv.org/abs/hep-ph/0011060, on the BTSM forum. Hanssen asserts that because the Lagrangian of QCD is nonlinear, since the structure constants for su(3) do not vanish, therefore you can't do Fourier transforms on the fields, and hence can't define the ladder or number operators, so no particles, only interacting fields. He uses this to show why quarks cannot exist in isoloation (the supposed quark in the source of a color field, which, by nonlinearity is also itself the source of a color field, ... Thus the quark is always a nexus of intracting fields and cannot be isolated. What does anybody here think of this line of argument?