We suggest to classify baryon resonances as single-quark states in a mean
field, and/or as its collective excitations. Identifying the Roper resonance
N(1440), the nucleon resonance N(1535), and the singlet hyperon Lambda(1405) as
single-quark excitations, we find that there must be an exotic S=+1 baryon
resonance Theta+ (the "pentaquark") with a mass about 1440+1535-1405=1570 MeV
and spin-parity one-half-plus. We argue that Theta+ is an analog of the
Gamov--Teller excitation long known in nuclear physics.Comment: 9 p., 5 fig