population principle, now called the Malthusian principle. Freud also wrote on principles, especially the reality principle necessary to keep the id and pleasure principle in check. Biologists use the principle of priority and principle of Binomial nomenclature for precision in naming species. There are many principles observed in physics, notably in cosmology which observes the mediocrity principle, the anthropic principle, the principle of relativity and the cosmological principle. Other well-known principles include the uncertainty principle in quantum mechanics and the pigeonhole principle and superposition principle in mathematics. As axiom or logical fundament Principle of sufficient reason The principle states that every event has a rational explanation. The principle has a variety of expressions, all of which are perhaps best summarized by the following: For every entity x, if x exists, then there is a sufficient explanation for why x exists. For every event e, if e occurs, then there is a sufficient explanation for why e occurs. For every proposition p, if p is true, then there is a sufficient explanation for why p is true. However, one realizes that in every sentence there is a direct relation between the predicate and the subject. To say that "the Earth is round", corresponds to a direct relation between the subject and the predicate. Principle of non-contradiction According to Aristotle, “It is impossible for the same thing to belong and not to belong at the same time to the same thing and in the same respect.” For example, it is not possible that in exactly the same moment and place, it rains and does not rain. Principle of excluded middle The principle of the excluding third or "principium tertium exclusum" is a principle of the traditional logic formulated canonically by Leibniz as: either A is B or A isn't B. It is read the following way: either P is true, or its denial ¬P is. It is also known as "tertium non datur" ('A third (thing) is not'). Classically it is considered to be one of the most important fundamental principles or laws of thought (along with the principles of identity, non-contradiction and sufficient reason). See also Axiom Corollary Deduction Law (principle) Logical consequence Self-Evidence References External links Actus Essendi and the Habit of the First Principle in Thomas Aquinas (New York: Einsiedler Press, 2019). Concepts in epistemology Concepts in ethics Concepts in metaphysics Concepts in the philosophy of mind Concepts in