Space is the boundless, three-dimensional extent in which objects and events occur and have relativeposition and direction. Physical space is often conceived in three linear dimensions, althoughmodern physicists usually consider it, with time, to be part of the boundless four-dimensionalcontinuum known as spacetime. In mathematics one examines spaces with different numbers ofdimensions and with different underlying structures. The concept of space is considered to be offundamental importance to an understanding of the physical universe although disagreement continuesbetween philosophers over whether it is itself an entity, a relationship between entities, or part of aconceptual framework.Debates concerning the nature, essence and the mode of existence of space date back to antiquity;namely, to treatises like the Timaeus of Plato, in his reflections on what the Greeks called: chora /Khora (i.e. space), or in the Physics of Aristotle (Book IV, Delta) in the definition of topos (i.e. place),or even in the later geometrical conception of place as space qua extension in the Discourse on Place(Qawl fi al-makan) of the 11th century Arab polymath Ibn al-Haytham (Alhazen). Many of theseclassical philosophical questions were discussed in the Renaissance and then reformulated in the 17thcentury, particularly during the early development of classical mechanics. In Isaac Newtons view,space was absolute - in the sense that it existed permanently and independently of whether there wereany matter in the space. Other natural philosophers, notably Gottfried Leibniz, thought instead thatspace was a collection of relations between objects, given by their distance and direction from oneanother. In the 18th century, the philosopher and theologian George Berkeley attempted to refute thevisibility of spatial depth in his Essay Towards a New Theory of Vision. Later, the metaphysicianImmanuel Kant said neither space nor time can be empirically perceived, they are elements of asystematic framework that humans use to structure all experiences. Kant referred to space in hisCritique of Pure Reason as being: a subjective pure a priori form of intuition, hence it is anunavoidable contribution of our human faculties.In the 19th and 20th centuries mathematicians began to examine non-Euclidean geometries, in whichspace can be said to be curved, rather than flat. According to Albert Einsteins theory of generalrelativity, space around gravitational fields deviates from Euclidean space. Experimental tests ofgeneral relativity have confirmed that non-Euclidean space provides a better model for the shape ofspace.