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 "Je pense, donc je suis" (I think, therefore I am, or Cogito ergo sum), which he indicated in his Discourse on the Method was "the first principle of the philosophy of which I was in search." Descartes describes the concept of a first principle in the following excerpt from the preface to the Principles of Philosophy (1644): In physics 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 empirical model and fitting parameters. For example, calculation of electronic structure using Schrödinger's equation within a set of approximations that do not include fitting the model to experimental data is an ab initio approach. See also Abstraction Brute fact Law of thought Present Clean room implementation Primitive notion References Further reading Orestes J. Gonzalez, Actus Essendi and the Habit of the First Principle in Thomas Aquinas (New York: Einsiedler Press, 2019). Abstraction Principles Foundationalism Formal systems