But then, this work at North Carolina, on the scattering matrix and on resonating group structure, left a lot of room open for what one said about the force between two of the elementary particles that made up the nuclei. Because, clearly, the interaction between two groups of particles was a consequence of many individual interactions between individual particles. What was the force between one individual particle and another? And that was, to me, especially intriguing, because I could see that the features in the interaction between two particles were in many ways similar to those between two groups of particles. But I had already recognized that in the interaction between two groups of particles, one could not imagine the description leading to a force alone that depended only on position between the particles, it must also depend on their velocities. And whatever was a feature of the groups of particles, I thought must also belong to particles themselves, so I introduced the concept of Velocity Dependent Forces. Well, we still have no significant drive from the experimental side, that forces us to conceive of velocity dependent forces between elementary particles, so that's still an open door.