In this essay we discuss epistemological implications of relativistic quantum ?eld theory. The empirical domain of such a theory is formed by phenomena ascribed tosubnuclearparticles, sometimes still calledelementaryparticles. Thislattermoretraditionaldesignationre?ectsthelastingdesireofphysicists to eventually ?nd and isolate irreducible constituents of matter. Going down to the atomic level, electrons appear to play such a role, whereas the nuclei of atomscanbeconsideredascompoundsystemsofprotonsandneutrons, i. e. of two species of particles. This view makes sense, since the respective...