Nambu, Y.; A Systematics of Hadrons in Subnuclear Physics
Preludes in Theoretical Physics in Honor of V.~F.~Weisskopf, ed. by A.~De-Shalit, H.~Feshbach, and L.~Van~Hove, North-Holland, Amsterdam (1966) 133 (1966);

Motivation
With the recognition that the SU(3) symmetry is the dominant feature of the strong interactions, the main concern of the elementary particle theory has naturally become directed at the understanding of the internal symmetry of particles at a deeper level. An immediate question that arises in this regard is whether there are fundamental objects (such as triplets or quartets) of which all the known baryons and mesons are composed. These fundamental objects would be to the baryons and mesons what
the nucleons are to the nuclei, and the electrons and nuclei are to the atoms. If that was really the case, it would certainly precipitate a new revolution in our conceptual image of the world. At the moment we can only hope that the question will be answered within the next ten to twenty years when the 100 GeV to 1000 GeV range accelerators will have been realized. Even now, the amusing and rather embarrassing success of the SU(6) theory ends support to the existence of those fundamental objects.
It is embarassing because this is basically a non-relativistic and static theory, and we do not know exactly how this can cover the realm of high energy relativistic phenomena. Putting aside those theoretical difficulties mainly associated with relativity, let us make the working hypothesis that there are fundamental objects which are heavy (>> 1 GeV), though not necessarily stable, and that inside each baryon or meson they are combined with a large binding energy, yet moving with non-relativistic
velocities. Though this might look like a contradiction, at least it does not violate the uncertainty principle in non-relativistic quantum mechanics since the range of the binding forces (10^{-14} - 10^{-13} cm) is large compared to the Compton wave lengths of those constituents, and the strength of the forces can be arbitrarily adjusted. In other words, we have a model very similar to the atomic nuclei except for large binding energies. Theoretical justification of such a hypothesis
must await future investigation. ... ... The SU(6) symmetry can be brought in, with the Pauli principle taken into account, since the constituent particles are non-relativistic. In another paper, we also considered a three-triplet model, in which t_{1}, t_{2} and t_{3} have charge assignments (1,0,0), (1,0,0) and (0, -1, -1) respectively. This has the advantage that the baryon states (the 56-dimensional representation of SU(6)) may be realized with s-state triplets as
~ t_{1}t_{2}t_{3}. The reasoning that has gone into the above stability problem is similar to the one used in nuclear physics in deriving the semi-empirical formula of Wiezsäcker. The purpose of the present paper is to put this idea into a more precise form, even though the outcome should still be called at best semi-quantitative. (Extracted from the introductory part of the paper.).

Related references See also F. Gürsey and L. A. Radicati, Phys. Rev. Lett. 13 (1964) 173;
A. Pais, Phys. Rev. Lett. 13 (1964) 175;
B. Sakata, Phys. Rev. 136 (1964) B1765;
J. G. Belinfante and R. E. Cutkosky, Phys. Rev. Lett. 14 (1965) 33;
M. Y. Han and Y. Nambu, Syracuse University preprint 1206-SU-31;
R. Capps, Phys. Rev. Lett. 14 (1965) 31;
Y. Nambu, Proc. of the Second Coral Gables Conference on Symmetry Principles at High Energy, University of Miami, (Jan. 1965);

Record comments
Invention of the idea of a vector gluon theory for strong interactions.