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In [[mathematics]], [[logic]], and [[formal system]]s, 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 (knowledge)|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]].
==Details==
[[Alfred Tarski]] explained the role of primitive notions as follows:<ref>[[Alfred Tarski]] (1946) ''Introduction to Logic and the Methodology of the Deductive Sciences'', page 118, [[Oxford University Press]].</ref>
: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,...
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