Sleep has been seen in culture as similar to death since antiquity; in Greek mythology, Hypnos (the god of sleep) and Thanatos (the god of death) were both said to be the children of Nyx (the goddess of night). John Donne, Samuel Taylor Coleridge, Percy Bysshe Shelley, and other poets have all written poems about the relationship between sleep and death. Shelley describes them as "both so passing, strange and wonderful!" Many people consider dying in one's sleep the most peaceful way to die. Phrases such as "big sleep" and "rest in peace" are often used in reference to death, possibly in an effort to lessen its finality. Sleep and dreaming have sometimes been seen as providing the potential for visionary experiences. In medieval Irish tradition, in order to become a filí, the poet was required to undergo a ritual called the "imbas forosnai", in which they would enter a mantic, trancelike sleep.