It is a pity to see the number of people who are eager for peace of mind. Some people take classes to attain peace of mind, some take drugs, some try meditation and other techniques, all of which are a waste of time if the goal is to attain peace of mind. A fundamental principle of Kabbalah is that peace of mind is actually quite unattainable during one's lifetime because the human soul has two aspects:

• Yetzer Ha'Tov (the good urge)
• Yetzer Ha'Ra (the bad urge)

These two aspects of the human soul are in constant battle with each other. However, the evil urge, the Yetzer Ha'Ra is not really evil in and of itself. The great sages tell the story that when some people decided to capture Yetzer Ha'Ra and lock it up in the prison, the result was that the houses were not built and babies were not born. This metaphorical story implies that the Yetzer Ha'Ra isn't actually evil but rather an impulse (Yetzer) that may be used for constructive or destructive purposes.

There is no way to stop the battle between the Yetzer Ha'Tov and the Yetzer Ha'Ra, nor would one want to. The goal of life is to use the battle between the two urges as a source of creativity. The tension between these two urges and the ultimate victory of one of the other (the good over the evil) results in the repairing and the building of the world. The goal isn't to end the battle but rather to raise it to the highest level possible, where the strife that the soul experiences results in purification, ethical living, acts of lovingkindness, and profound connection to God.
-- Kabbalah for Dummies, ArthurKurzweil.