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Space is the continuum, measured in units of length, within which matter is physically extended and objects have positions relative to one another. In the physical world space has three dimensions, although modern physicists usually consider it, with time, to be part of the boundless four-dimensional extent known as spacetime. In mathematics spaces with different numbers of dimensions and with different underlying structures can be examined. The concept of space is considered to be of fundamental importance to an understanding of the universe but its exact nature is the subject of debate amongst philosophers.

Many of the philsophical questions arose in the 17th century during the early development of classical mechanics. In Isaac Newton's view space was absolute, in the sense that it existed permanently and independently of whether there was any matter in the space or moving through it. Other natural philosophers, notably Gottfried Leibniz, thought instead that space was a collection of relations between objects, given by their distance and direction from one another. In the 18th century Immanuel Kant described space and time as elements of a systematic framework which humans construct to structure their experience. He believed that the properties of space could be determined a priori using the rules of geometry set out by Euclid in the 3rd century BC.

In the 19th and 20th centuries mathematicians began to examine non-Euclidean geometries, in which space can be said to be curved, rather than flat. According to Albert Einstein's theory of general relativity space has a structure that deviates in gravitational fields from the structure of Euclidean geometry. Experimental tests have confirmed that, for the existing laws of mechanics and optics to hold, space must be described by non-Euclidean geometry.