Hi there, I have a question about the rest mass of an electron. As we all know, the charge of an electron is a function of the energy at which the system is probed. When defining the charge, we typically use as our reference scale the charge measured in Thompson scattering at the orders of the electron mass. However, the mass of the electron is itself a function of the energy -- partly because, in an interacting theory, the mass measures the strength of self-interactions. So how do we define *it*? We can't say 'the mass of an electron at the order of the mass of an electron', because that would be circular. Can anyone help me here?! In particular, is there a lowest energy scale that is appropriate for defining the mass of an electron? Any help much appreciated!