Aletheia or Alethia (ælɪ'θaɪ.ə; ἀλήθεια) is truth or disclosure in philosophy. Originating in Ancient Greek philosophy, the term was explicitly used for the first time in the history of philosophy by Parmenides in his poem On Nature, in which he contrasts it with doxa.
It was revived in the works of 20th-century philosopher Martin Heidegger. Although often translated as "truth", Heidegger argued that it is distinct from common conceptions of truth.
is variously translated as "unconcealedness", "disclosure", "revealing", or "unclosedness". The literal meaning of the word ἀ–λήθεια is "the state of not being hidden; the state of being evident." It also means factuality or reality. It is the antonym of , which literally means "oblivion", "forgetfulness", or "concealment" according to Pindar's First Olympian Ode.
In Greek mythology, was personified as a Greek goddess, Aletheia. In some accounts she was a daughter of Zeus, while Aesop's Fables state she was crafted by Prometheus. In she was equated with Veritas, the Roman goddess of truth.
In the early to mid 20th-century, Martin Heidegger brought renewed attention to the concept of aletheia, by relating it to the notion of disclosure, or the way in which things appear as entities in the world. While he initially referred to aletheia as "truth", specifically a form that is pre-Socratic in origin, Heidegger eventually corrected this interpretation, writing:
Aletheia, disclosure ("Unverborgenheit"), regarded as the opening (Lichtung) of presence ("Anwesenheit") is not yet truth ("Wahrheit"). Is therefore aletheia something less than truth? Or is it more because it first grants truth as adaequatio and certitudo, because there can be no presence and presenting outside of the realm of the opening? (...) To raise the question of aletheia, of disclosure as such, is not the same as raising the question of "truth". For this reason, it was inadequate and misleading to call aletheia, in the sense of opening, truth.
Heidegger gave an etymological analysis of aletheia and drew out an understanding of the term as "unconcealedness".