A first principle is a basic proposition or assumption that cannot be deduced from any other proposition or assumption.
In philosophy, first principles are from First Cause attitudes and taught by Aristotelians, and nuanced versions of first principles are referred to as postulates by Kantians.
In mathematics, first principles are referred to as axioms or postulates.
In physics and other sciences, theoretical work is said to be from first principles, or ab initio, if it starts directly at the level of established science and does not make assumptions such as empirical model and parameter fitting.
“I think it’s important to reason from first principles rather than by analogy. The normal way we conduct our lives is we reason by analogy.” - Elon Musk