Λήθη

Lēthē comes from the verb lēthein or lanthanein, meaning 'to escape notice' or 'to forget.' The river Lethe flowed through the underworld, and the dead drank from it to erase their memories before reincarnation. Plato describes the process in the Republic (Book 10, ~375 BCE): souls choose their next life, then drink from the River of Forgetfulness on the Plain of Oblivion. Those who drink too much forget everything. Those who are wise drink sparingly.

The word's most surprising relative is alētheia — Greek for 'truth.' The a- is a negation. Alētheia is literally 'un-forgetting' or 'un-concealment.' For the Greeks, truth was not something you constructed; it was something you stopped hiding. Martin Heidegger built a large part of his philosophy on this etymology, arguing in the 1930s and 1940s that the Greeks understood truth as disclosure — the opposite of lethe. The river of forgetting and the concept of truth are mirror images of each other.

The adjective 'Lethean' entered English by the early 17th century, meaning 'causing forgetfulness.' Keats used it in 'Ode to a Nightingale' (1819): 'My heart aches, and a drowsy numbness pains / My sense, as though of hemlock I had drunk, / Or emptied some dull opiate to the drains / One minute past, and Lethe-wards had sunk.' The river became a poetic synonym for oblivion, for the mercy of not remembering. Some scholars have also proposed a connection between lēthē and the word 'lethal' — from Greek lēthalis — but this derivation is disputed.

The chemical element lanthanum (La, atomic number 57), discovered in 1839 by Carl Gustaf Mosander, takes its name from the same root: lanthanein, to lie hidden. The element had been concealed inside cerium oxide, escaping notice until Mosander isolated it. A Greek verb about forgetting became the name of a metal that was hard to find. Meanwhile, latent (from Latin latēre, from the same PIE root *leh₂- 'to be hidden') entered English from a different branch of the same family. Forgetting, hiding, and truth are all one word.