In mathematics, logic, and formal systems, a primitive notion is an undefined concept. In particular, a primitive notion is not defined in terms of previously defined concepts, but is only motivated informally, usually by an appeal to intuition and everyday experience. In an axiomatic theory or other formal system, the role of a primitive notion is analogous to that of axiom. In axiomatic theories, the primitive notions are sometimes said to be "defined" by one or more axioms, but this can be misleading. Formal theories cannot dispense with primitive notions, under pain of infinite regress.
Alfred Tarski explained the role of primitive notions as follows:
- When we set out to construct a given discipline, we distinguish, first of all, a certain small group of expressions of this discipline that seem to us to be immediately understandable; the expressions in this group we call PRIMITIVE TERMS or UNDEFINED TERMS, and we employ them without explaining their meanings. At the same time we adopt the principle: not to employ any of the other expressions of the discipline under consideration, unless its meaning has first been determined with the help of primitive terms and of such expressions of the discipline whose meanings have been explained previously. The sentence which determines the meaning of a term in this way is called a DEFINITION,...
- [The] 'definition' of 'set' is less a definition than an attempt at explication of something which is being given the status of a primitive, undefined, term.
As evidence, she quotes Felix Hausdorff: "A set is formed by the grouping together of single objects into a whole. A set is a plurality thought of as a unit."
When an axiomatic system begins with its axioms, the primitive notions may not be explicitly stated. Susan Haak (1978) wrote, "A set of axioms is sometimes said to give an implicit definition of its primitive terms."
- Naive set theory, the empty set is a primitive notion. (To assert that it exists would be an implicit axiom.)
- Peano arithmetic, the successor function and the number zero are primitive notions.
- Axiomatic systems, the primitive notions will depend upon the set of axioms chosen for the system. This was discussed by Alessandro Padoa at the International Congress of Mathematicians in Paris in 1900.
- Euclidean geometry, under Hilbert's axiom system the primitive notions are point, line, plane, congruence, betweeness and incidence.
- Euclidean geometry, under Peano's axiom system the primitive notions are point, segment and motion.
- Philosophy of mathematics, Bertrand Russell considered the "indefinables of mathematics" to build the case for logicism in his book The Principles of Mathematics (1903).
 See also
- Axiomatic set theory
- Foundations of mathematics
- Mathematical logic
- Notion (philosophy)
- Object theory