Eternity in common parlance is either an infinite or an indeterminately long period of time. However, in classical philosophy, eternity is held to be that which exists outside of time with sempiternity being the concept that corresponds to the colloquial definition. Eternity, or everlasting, is an important concept in many religions, where the immortality of God or the gods is said to endure eternally. Some, such as Aristotle, would say the same about the natural cosmos in regard to both past and future eternal duration, and like the eternal Platonic Forms, immutability was considered essential.
The metaphysics of eternity studies that which necessarily exists outside or independently of space and time. Another important question is whether "information" or Form is separable from mind and matter. Aristotle established a distinction between actual infinity and a potentially infinite count: a future span of time must be a potential infinity, because another element can always be added to a series that is inexhaustible.
Aristotle argued that the cosmos has no beginning. Euclid invoked this distinction instead of saying that there are an infinity of primes, rather that the primes outnumber those contained in any given collection thereof.
In Aristotle's metaphysics, eternity is the unmoved mover (God), understood as the gradient of total synergy ("produces motion by being loved"). Boethius defined eternity as "simultaneously full and perfect possession of interminable life".
Eternity is often symbolized by the image of a snake swallowing its own tail, known as the Ouroboros (or Uroboros). The circle is also commonly used as a symbol for eternity, as is the mathematical symbol of infinity, <math>\infty</math>.
Eternity is mentioned in religious texts.
- The Quran states:
Gardens of Eternity, whose doors will (ever) be open to them. Sad 38:50
- The Dhammapada states:
Hatred is never appeased by hatred in this world. By non-hatred alone is hatred appeased. This is a law eternal. Dhammapada, The pairs of Heedfulness 
- The Bhagavad Gita states:
The soul never takes birth and never dies at any time nor does it come into being again when the body is created. The soul is birthless, eternal, imperishable, and timeless and is never destroyed when the body is destroyed. 2:20
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- Eternalism (philosophy of time)
- Presentism (philosophy)
- Eternal return
- Perennial philosophy
- Temporal finitism
- Sense of time
- Timeline of the Big Bang
- Planck epoch
- Yin and yang
- Stanford Encyclopedia of Philosophy, Eternity
- "For generally the infinite has this mode of existence: one thing is always being taken after another, and each thing that is taken is always finite, but always different". Aristotle, Physics, Book III, Chapter VI
- Elements, Book IX, Proposition 20
- Yu, Jiyuan The Structure of Being in Aristotle’s Metaphysics Springer, 2003, p. 188
- Boedder, Bernard. "Natural Theology". University of Notre Dame. Retrieved 15 March 2015.
Aeternitas est interminablis vitae tota simul et perfecta possessio
- Gautam, Neeraj (2009). Buddism a Faith of World. Pinnacle Technology. ISBN 9781618202031.
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- http://plato.stanford.edu/entries/eternity/ Entry in the Stanford Encyclopedia of Philosophy on Eternity.
- http://www.iep.utm.edu/g/god-time.htm Entry in the Internet Encyclopedia of Philosophy on the relationship between God and Time.
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