A first principle is one that cannot be deduced from any other. The classic example is that of Euclid's (see Euclid's Elements) geometry; its hundreds of propositions can be deduced from a set of definitions, postulates, and common notions: all three of which constitute "first principles."
and in English translation:
This principle is the first expression of consistency in western thought. Any defining and reasoning in any language on any topic assumes it a priori. It cannot be doubted, as all doubting is based on inconsistency, which assumes consistency a priori.
Profoundly influenced by Euclid, Descartes, the "father of modern philosophy", was a rationalist who invented the foundationalist system of philosophy. He used the "method of doubt", now called Cartesian doubt, to systematically doubt everything he could possibly doubt, until he was left with what he saw as purely indubitable truths. Using these self-evident propositions as his "axioms", or "foundations", he went on to deduce his entire body of knowledge from them. (The foundations are also called a priori truths.) His most famous proposition is "I think, therefore I am", or "Cogito ergo sum".
In physics, a calculation is said to be from first principles, or ab initio, if it starts directly at the level of established laws of physics and does not make assumptions such as model and fitting parameters.
US Patent Issued to Emerson Process Management Power and Water Solutions on Oct. 15 for "Methods and Apparatus to Compensate First Principle-Based Simulation Models" (Pennsylvania Inventors)
Oct 15, 2013; ALEXANDRIA, Va., Oct. 15 -- United States Patent no. 8,560,283, issued on Oct. 15, was assigned to Emerson Process Management...