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Absolute geometry is a geometry based on an axiom system consisting of all the axioms giving Euclidean geometry except for the parallel postulate or any of its alternatives. [69] The term was introduced by János Bolyai in 1832. [70] It is sometimes referred to as neutral geometry, [71] as it is neutral with respect to the parallel postulate.
Although these terms are not further defined, Euclid uses them to construct more complex geometric concepts. [5] Whether a particular function or value is undefined, depends on the rules of the formal system in which it is used. For example, the imaginary number is undefined within the set of real numbers.
Alfred Tarski explained the role of primitive notions as follows: [4]. 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.
In Euclidean geometry, similarity is used to describe objects that have the same shape, while congruence is used to describe objects that are the same in both size and shape. [69] Hilbert, in his work on creating a more rigorous foundation for geometry, treated congruence as an undefined term whose properties are defined by axioms.
In other words, the elements of geometry form a system which is not susceptible of extension, if we regard the five groups of axioms as valid. The old axiom V.2 is now Theorem 32. The last two modifications are due to P. Bernays. Other changes of note are: The term straight line used by Townsend has been replaced by line throughout.
Placing Euclidean geometry on a solid axiomatic basis was a preoccupation of mathematicians for centuries. [48] The role of primitive notions, or undefined concepts, was clearly put forward by Alessandro Padoa of the Peano delegation at the 1900 Paris conference: [48] [49]
When a geometry is described by a set of axioms, the notion of a line is usually left undefined (a so-called primitive object). The properties of lines are then determined by the axioms which refer to them. One advantage to this approach is the flexibility it gives to users of the geometry.
Euclid set forth the first great landmark of mathematical thought, an axiomatic treatment of geometry. [2] He selected a small core of undefined terms (called common notions) and postulates (or axioms) which he then used to prove various geometrical statements.