I have for the past year (longer, actually) been working on the idea that neither gravitational nor inertial mass are intrinsic, but are instead emergent, arising from the interaction of matter with the all-pervasive field of the quantum vacuum. There are tantalizing leads of this concept in Sakharov's work, but today (beginning to re-read "Genius" by James Gleick) I found this little gem (in the signature area at the bottom of the post). Dr. Feynman must have thought it important, since he wrote it in his "little black book" and kept it with him. I do not understand where Feynman was going with the first part of the notation, since statements about equivalence and substructure are common in physics, but "All mass is interaction." hits home. I believe that the Machian concept of inertial mass relative to a fixed reference frame is correct, but that the reference frame is not that of the sum of all the matter in the universe (which implies superluminal action at a distance) but that of the quantum vacuum - evanescent, constantly frothing, capable of densification and polarization (due to the presence of matter) but AT REST, since the virtual particle pairs pop in and out of existence in the time allowed by the Heisenberg uncertainty principle and cannot exist long enough to exhibit measurable movement (Planck time). In this respect, loop quantum gravity (although not as popular as "string" or M theory, if the numbers of papers are any guide) may be the most promising place to expect an explanation of gravitation relative to the truly fundamental forces.