Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of the boundless four-dimensional continuum known as spacetime. In mathematics one examines 'spaces' with different numbers of dimensions and with different underlying structures. The concept of space is considered to be of fundamental importance to an understanding of the physical universe although disagreement continues between philosophers over whether it is itself an entity, a relationship between entities, or part of a conceptual 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 these classical philosophical questions were discussed in the Renaissance and then reformulated in the 17th century, particularly 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 were any matter in the space. 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, the philosopher and theologian George Berkely attempted to refute the 'visibility of spatial depth' in his Essay Towards a New Theory of Vision. Later, the great metaphysician Immanuel Kant described space and time as elements of a systematic framework that humans use to structure their experience; he referred to 'space' in his Critique of Pure Reason as being: a subjective 'pure a priori form of intuition', hence that its existence depends on our human faculties.

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 around gravitational fields deviates from Euclidean space. Experimental tests of general relativity have confirmed that non-Euclidean space provides a better model for the shape of space.

Space is the relatively empty space between celestial bodies such as stars, planets and moons.

• "Beyond a critical point within a finite space, freedom diminishes as numbers increase. ...The human question is not how many can possibly survive within the system, but what kind of existence is possible for those who do survive." - Dune by Frank Herbert

• "Einstein's space is no closer to reality than Van Gogh's sky. The glory of science is not in a truth more absolute than the truth of Bach or Tolstoy, but in the act of creation itself. The scientist's discoveries impose his own order on chaos, as the composer or painter imposes his; an order that always refers to limited aspects of reality, and is based on the observer's frame of reference, which differs from period to period as a Rembrandt nude differs from a nude by Manet." - The Act of destruction by Arthur Koestler

• "I have sat by night beside a cold lake

And touched things smoother than moonlight on still water,

But the moon on this cloud sea is not human,

And here is no shore, no intimacy,

Only the start of space, the road to suns." - Trans Canada by F.R. Scott

• "Interestingly, according to modern astronomers, space is finite. This is a very comforting thought-- particularly for people who can never remember where they have left things." - Woody Allen

• "No creature loves an empty space;

Their bodies measure out their place" - Upon Appleton House, to My Lord Fairfax by Andrew Marvell

• "Once the threshold is crossed when there is a self-sustaining level of life in space, then life's long-range future will be secure irrespective of any of the risks on Earth (with the single exception of the catastrophic destruction of space itself). Will this happen before our technical civilisation disintegrates, leaving this as a might-have-been? Will the self-sustaining space communities be established before a catastrophe sets back the prospect of any such enterprise, perhaps foreclosing it for ever? We live at what could be a defining moment for the cosmos, not just for our Earth." ~ Our Final Hour by Martin Rees

• "Since, in the long run, every planetary society will be endangered by impacts from space, every surviving civilization is obliged to become spacefaring--not because of exploratory or romantic zeal, but for the most practical reason imaginable: staying alive." ~ Pale Blue Dot by Carl Sagan

• "Space is big. Really big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space." - The Hitchhiker's Guide to the Galaxy by Douglas Adams

• "Space is the breath of art." - Frank Lloyd Wright

• "Space is to place as eternity is to time." - Joseph Joubert

• "Space, like time, engenders forgetfulness; but it does so by setting us bodily free from our surroundings and giving us back our primitive, unattached state" - Thomas Mann

• "Space... the final frontier. These are the voyages of the starship Enterprise. Its continuing mission, to explore strange new worlds, to seek out new life and new civilizations, to boldly go where no one has gone before!" - Star Trek: The Next Generation (Opening Credits) attrib Gene Roddenberry

• "Do all the stars have systems of planets?" - Anakin Skywalker
• "When we contemplate the whole globe as one great dewdrop, striped and dotted with continents and islands, flying through space with other stars all singing and shining together as one, the whole universe appears as an infinite storm of beauty." - Travels in Alaska by John Muir

• "People ask if there is life out there, but why is it that the only people get abducted are morons. Why would intelligent life with advanced technology come out to Earth and pick out a drunk idiot that nobody believes in the first place. I think Earth is the Alabama of space and aliens just happen on Earth just like normal people end up in Alabama. - Michael Stoneman

• "Space. It seems to go on and on forever. Then you get to the end, and a monkey starts throwing barrels at you." - Phillip Fry, Futurama