The concept of twin siblings is frequently explored in the literature and mythologies of many cultures. In some, they are seen as ominous and in others, they are auspicious. Twins in mythology are often cast as two halves of the same whole, sharing a bond deeper than that of ordinary siblings, or otherwise shown as fierce rivals. Twins can represent some other aspect of the "Self"; a doppelganger or "shadow." Often the twin is the "evil twin"[citation needed], or one may be human and one semi-divine. The twin may be a brother, or a soul-mate, such as the "civilized" Gilgamesh and the "wild" Enkidu. Twins can represent the dualistic nature of the universe. In Greek mythology, Apollo and Artemis are twins, and Apollo was adopted as the sun god with Artemis as the moon goddess. In one version of the Egyptian creation myth, the earth god Geb and the sky goddess Nut were twins. Twins can also be shown as having special powers and deep bonds. In Greek mythology, Castor and Pollux share a bond so strong that when Castor dies, Pollux gives up half of his immortality to be with his brother. In literature, a "doppelganger" is often revealed to be a twin; a shadow or mirror image of the protagonist. The double motif involves a comparison or contrast between two characters or sets of characters within a work to represent opposing forces in human nature. For example, Dr. Jekyll and his evil double Mr. Hyde are contrasted to represent the battle between the rational, intellectual self (Jekyll) and the irrational, bestial self (Hyde). The double might represent repressed emotion and acting out the forbidden desire of the main character just like in Jane Eyre where the madwoman in the attic serves as Jane's darkest double, representing her complete irrationality or suppressed rage against confined life of a governess. In "Great Expectations," where Orrick represents Pip's double and both are cut off from what they want, but because Pip remains rational he can save himself. Orlick does what Pip secretly wishes, gets rid of his sister by using file Pip used to free Magwitch at the beginning of the novel. Furthermore, it may be figured as one person existing in two different places at the sam