<P> Euclid described a line as "breadthless length" which "lies equally with respect to the points on itself"; he introduced several postulates as basic unprovable properties from which he constructed all of geometry, which is now called Euclidean geometry to avoid confusion with other geometries which have been introduced since the end of the 19th century (such as non-Euclidean, projective and affine geometry). </P> <P> In modern mathematics, given the multitude of geometries, the concept of a line is closely tied to the way the geometry is described . For instance, in analytic geometry, a line in the plane is often defined as the set of points whose coordinates satisfy a given linear equation, but in a more abstract setting, such as incidence geometry, a line may be an independent object, distinct from the set of points which lie on it . </P> <P> 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 . Thus in differential geometry a line may be interpreted as a geodesic (shortest path between points), while in some projective geometries a line is a 2 - dimensional vector space (all linear combinations of two independent vectors). This flexibility also extends beyond mathematics and, for example, permits physicists to think of the path of a light ray as being a line . </P> <P> All definitions are ultimately circular in nature since they depend on concepts which must themselves have definitions, a dependence which cannot be continued indefinitely without returning to the starting point . To avoid this vicious circle certain concepts must be taken as primitive concepts; terms which are given no definition . In geometry, it is frequently the case that the concept of line is taken as a primitive . In those situations where a line is a defined concept, as in coordinate geometry, some other fundamental ideas are taken as primitives . When the line concept is a primitive, the behaviour and properties of lines are dictated by the axioms which they must satisfy . </P>

What is one way that a line is the same as a ray
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