I am a theorist and I would say, that the bottom - as it is the foundation - is mathematical logic. And you start from there. Baffingly enough: All this theory

I am a theorist and I would say, that the bottom - as it is the foundation - is mathematical logic. And you start from there. Baffingly enough: All this theory is easily understood with 7th-grade math. Approaching from some specific piece of engineering is surely top-down.