The first principles here refer to the basic equations of the Standard Model. There were a lot of alternative theories in sixties and seventies but eventually t

The first principles here refer to the basic equations of the Standard Model. There were a lot of alternative theories in sixties and seventies but eventually they were disproven experimentally and only SM survived. But the problem with SM is that when one deals with strong interactions, nobody so far came with good ways to calculate things. For example, in theory the mass of proton can be derived. But in practice it is not. So one uses that as an extra parameter and tries to use that to constrain calculations in other areas.