In general, absolute truth is whatever is always valid, regardless of parameters or context. The absolute in the term connotes one or more of: a quality of truth that cannot be exceeded; complete truth; unvarying and permanent truth. It can be contrasted to relative truth or truth in a more ordinary sense in which a degree of relativity is implied.
1) In philosophy, absolute truth generally states what is essential rather than superficial - a description of the Ideal (to use Plato's concept) rather than the merely "real" (which Plato sees as a shadow of the Ideal). Among some religious groups this term is used to describe the source of or authority for a given faith or set of beliefs, such as the Bible.
2) In science, doubt has been cast on the notion of absolutes by theories such as relativity and quantum mechanics . Attempts to tie together all the known facts about the universe into a single unified theory (one example is string theory ) could be seen as efforts to discover absolute truth about this set of facts.
3) In pure mathematics , however, there is said to be a proof for the existence of absolute truth. A common tactic in mathematical proofs is the use of reductio ad absurdum , in which the statement to be proved is denied as a premise, and then that premise is shown to lead to a contradiction. When it can be demonstrated that the negation of a statement leads to a contradiction, then the original statement is proved true.
The logical proof of the statement, "There exists an absolute truth," is almost trivial in its simplicity. Suppose we assert the negation of the statement, that is, that there is no such thing as absolute truth. By making that assertion, we claim that the sentence "There exists no absolute truth" is absolutely true. The statement is self-contradictory, so its negation, "There exists an absolute truth," is true.
This proof applies only to logic. It does not tell us whether any particular statement other than itself is true. It does not prove the existence (or non-existence) of God, the devil, heaven, hell, or little green people from another galaxy. Neither does it assert that we can always ascertain the truth or falsity of any arbitrary statement. The Incompleteness Theorem , proved by Kurt Gödel and published in 1931, actually showed that there exist logical statements whose truth value is undecidable, that is, they cannot be proved either true or false.