Elisha Ben Avuya was dubbed Acher, "other," because he left the side of Torah (the side of faith) and went to the "other side" (the side of heresy).
In sleep one turns from side to side. The more uncomfortable one is the more one turns sides, seeking rest on the other side. As things become more and more intolerable, the sick man, half awake half asleep, begins to see things upside down, to turn around reality.
Isaiah describes this sad state in the following words: "Woe unto them that call evil good, and good evil; that put darkness for light, and light for darkness; that put bitter for sweet, and sweet for bitter."
You don't have to know that you're sick to be sick (in truth, knowledge of one's illness is half its cure). So was the case of Elisha Acher.
Let's do some numerical surgery on Elisha (first putting him to sleep on the operation table). His name, Elisha (which literally reads אלי שע, "My God turn [to me]"!) equals 411, the value of the 10th word of the Torah – תהו, "chaos" ("And the earth was chaos…"). In Kabbalah, the primordial World of Chaos is an unstable spiritual state, a world of great lights with small, immature vessels, unable to contain the lights. The intensity of the lights breaks the vessels. In this world, a soul from the World of Chaos never finds rest; he is constantly turning over from side to (other) side.
In terms of the ten sefirot, the Divine emanations that serve as channels of creation, chaos is identified with the highest of the sefirot, the super-rational crown (keter). The numerical value of Elisha Acher, 620, is equal to that of כתר, "crown."
From this we may infer that to rectify Elisha's soul the lights of the supernal crown, the unconscious mind, must be drawn into the realm of consciousness, become rational and capable of distinguishing between good and evil, light and darkness, sweet and bitter.