In Judaism, yetzer hara (Hebrew: יֵצֶר הַרַע, for the definite "the evil inclination"), or yetzer ra (Hebrew: יֵצֶר רַע, for the indefinite "an evil inclination") refers to the congenital inclination to do evil, by violating the will of God. The term is drawn from the phrase "the imagination of the heart of man [is] evil" (Hebrew: יֵצֶר לֵב הָאָדָם רַע, yetzer lev-ha-adam ra), which occurs twice in the Hebrew Bible, at Genesis 6:5 and 8:21. The Sages of the Talmud (Berakhot 32a) have spoken about the "evil inclination" in poignant terms, making a comparison to what it is like: “To what is it like, the evil inclination in man? It is like a father who takes his small son, bathes him, douses him with perfume, combs his hair, dresses him up in his finest accoutrements, feeds him, gives him drink, places a bag of money around his neck, and then goes off and puts his son at the front door of a brothel. What can the boy do that he not sin?”
The evil inclination in man, or what is often called man's natural inclination, has been the subject of debate since time immemorial. The traditional Jewish view on this complex subject is well-defined in rabbinic literature. The yetzer hara is not a demonic force, but rather man's misuse of things the physical body needs to survive. Thus, the need for food becomes gluttony due to the yetzer hara. The need for procreation becomes sexual abuse, and so on. The idea that humans are born with a yetzer ra (physical needs that can become "evil"), but that humans don't acquire a yetzer tov ("a good inclination") until an age of maturity—12 for girls and 13 for boys—has its source in Chapter 16 of the Talmudic tractate Avot de-Rabbi Natan.
The underlying principle in Jewish thought states that every man is born with both a good inclination and an evil inclination. This, in itself, is not bad, nor is it an abnormality. The problem, however, arises when one makes a willful choice to "cross over the line," and seeks to gratify his "evil inclination," based on the prototypical models of right and wrong in the Hebrew Bible.
Central to Jewish belief is the idea that every man - Jew and gentile alike - is born with two opposing inclinations that pull him to act either in a bad way or a good way, but that, in the final analysis, it is man who decides how he is to act. This notion is succinctly worded in the Babylonian Talmud (Niddah 16b): "All is given into the hands of heaven, except one's fear of heaven," meaning, everything in man's life is pre-determined by God - excepting that man's choice to be either good or bad; righteous or wicked. In this matter alone, man must decide for himself whether he will choose good or bad, or what is often classified as a man's freewill. Traditionally, a person's indulgence of either the good or evil impulse is seen as a matter of free choice.
Most men will, at some time in their lives, succumb to their evil inclination, as it is written: “For there is not a righteous man upon earth, that doeth good, and sinneth not.”(Ecclesiastes 7:20). For this reason, repentance (and in some cases, affliction) is said to atone for most sins, while the preponderance of good works keeps him within the general class of good men. Medieval Rabbi and philosopher, Maimonides, has given instruction on how man is to view the Evil Inclination and ensuing hardships on that account:
Moshe Chaim Luzzatto wrote in Derech Hashem ("The Way of God") that "Man is the creature created for the purpose of being drawn close to God. He is placed between perfection and deficiency, with the power to earn perfection. Man must earn this perfection, however, through his own free will... Man's inclinations are therefore balanced between good (Yetzer HaTov) and evil (Yetzer HaRa), and he is not compelled toward either of them. He has the power of choice and is able to choose either side knowingly and willingly..."
While God has created man with both good and evil inclinations, the two powers or tendencies that pull him in opposite directions, God commands each man to choose the good and right path over the evil. In the biblical episode mentioning Cain and how he killed his brother, Abel, the narrative tells of God approaching Cain the murderer after this crime, and saying unto him [Genesis 4:7]: “Isn’t it so that if you do good, you shall be forgiven? However, if you will not do good it is because sin crouches at the entrance [of your heart], and to you shall be its longing, although you have the ability to subdue it.”
Medieval commentator Rashi explains: “and to you shall be its longing,” meaning, the longing of sin—i.e., the evil inclination—which constantly longs and lusts to cause you to stumble...“although you have the ability to subdue it,” meaning, if you wish, you will overpower it. [see: Sifrei on Deuteronomy, P. Ekev 45, Kidd. 30b].
The import is, therefore, clear that each man has the power within him to overcome sin if he really wishes to do so. This may not always be easy, but it is still possible. In some cases, it may take some reconditioning, but in the end, man is able to control his passions and/or abnormal behavior and to readjust by returning to what is considered the laws of good society.
Although some forms of Judaism, both ancient and modern, do recognise the existence of supernatural evil, in particular fallen angels (as in the Dead Sea scrolls), the yetzer hara is often presented as a personification of evil distinct from the supernatural Satan of traditional Christianity and Islam. This tendency to demythologize Satan is found in the Babylonian Talmud and other rabbinical works, e.g.: "Resh Laqish said: Satan, the evil inclination, and the Angel of Death are all one." Notably, however, this and other passages of the Talmud do not deny the external existence of Satan, but create a synthesis between external and internal forces of evil. Similar tendencies can also be found in some Enlightenment Christian writers, such as in the religious writings of Isaac Newton.