The Richtmyer memorial lecture—when is a particle?

Elementary particle concepts have evolved considerably in recent years. In particular, quarks are viewed as the basic constituents of subnuclear matter as probed by current experiments, although they themselves are not observed in isolation. This contrasts with earlier insistence on ’’seeing’’ our elementary particles, in particular the neutrino. The history of the neutrino is reviewed; the arguments for quarks are presented and discussed; the metaphysical origins of confined constituents in the writings of Greek and Roman philosophers are recalled; and the implications of permanently confined quarks for our view of elementary particles are considered.