Like many other classical unified field theorists, Eddington considered that in the Einstein field equations for general relativity the stress–energy tensor T μ μ ν ν , which represents matter/energy, was merely provisional, and that in a truly unified theory the source term would automatically arise as some aspect of the free-space field equations. He also shared the hope that an improved fundamental theory would explain why the two elementary particles then known (proton and electron) have quite different masses.