Immortality (or eternal life) is the concept of living in physical or spiritual form for an infinite length of time.

As immortality is the negation of mortality—not dying or not being subject to death—it has been a subject of fascination to mankind since at least the beginning of history. The Epic of Gilgamesh, one of the first literary works, dating back at least to the 22nd century BCE, is primarily a quest of a hero seeking to become immortal. What form an unending human life would take, or whether the soul exists and possesses immortality, has been a major point of focus of religion, as well as the subject of speculation, fantasy, and debate.

It is not known whether human physical immortality is an unachievable phenomenon or not. Biological forms have inherent limitations — for example, their fragility and slow adaptability to changing environments, which may or may not be able to be overcome through medical interventions, engineering, etc. On the other hand, biological immortality already exists among some simple, but multicellular life-forms.

Some scientists, futurists, and philosophers, such as scientists Aubrey de Grey and Ray Kurzweil believe that human immortality is achievable in the next few decades. Others are somewhere in the middle of these two extreme viewpoints, thinking that immortality is achievable in some period of time longer than 20-30 years, but not impossible.

Biological immortality is what life extension advocates feel is likely in decades to come. Specifically this refers to the abscence of aging of the body due to baseline biological human limitations, but acknowledgement that complete immortality in a human form is unlikely due to the fact that even when you remain biologically young, once every few hundred years individuals will get killed in an accident (4.4% of U.S. deaths now) or by other means .

Ultimately, a timeless existence is also not known for certain to be achievable, or even definable, despite millennia of arguments for eternity. Wittgenstein, in a notably non-theological interpretation of eternal life, writes in the Tractatus that, "If we take eternity to mean not infinite temporal duration but timelessness, then eternal life belongs to those who live in the present."

Spiritual

• The belief in an afterlife experienced by an immortal soul is a dogma of many branches of Christianity, Zoroastrianism, Hinduism, Islam, and Judaism. This belief is related to the philosophy of dualism.

Hypothetical

• Fame itself has been described as a method to "achieve immortality", if only semantically, so that the name or works of a famous individual would "live on" after his or her death. This view of immortality places value on how one will be remembered by generations to come. For example, in Homer's Iliad, Achilles is already nigh-invincible, so his primary motive for fighting in the Trojan War is recognition and everlasting fame.
• Mystic approaches to immortality include those of the ancient Chinese Taoists and European medieval alchemists, seeking an elixir of life.
• Should metaphysical universals and abstract phenomena have an eternal existence, and if they can be interacted with by human beings, then a person might obtain a degree of immortality by interacting with them.Fact: date=March 2008
• Quantum immortality is not widely regarded by the scientific community as being a verifiable or even necessarily correct offshoot of the many worlds interpretation. In the many worlds interpretation of quantum mechanics, the wavefunction never collapses, and thus all possible outcomes of a quantum event exist simultaneously, with each event apparently spawning an entirely new universe in which a single possible outcome exists. In this theory, a person could hypothetically live forever as there might exist a string of possible quantum outcomes in which that individual never dies.