The close analogy between the multiple quantum resonances observed in NMR and many-body effects in time and frequency domain nonlinear optical spectroscopy is explained. The nonlinear exciton Equations provide a systematic hierarchial method for incorporating and classifying collective effects beyond the mean-field level. The apparent inconsistencies pointed out by Jeener are caused by terminology differences which are clarified. (C) 2002 Elsevier Science B.V. All rights reserved.