Every Jew has a nefesh (life force) known as the Nefesh HaBahamis (Animal Soul).
In additional, there's the neshomah, also known as the Nefesh HoElokis (G‑dly Soul).
There is nothing bad about the Nefesh HaBahamis. The Nefesh HaBahamis is interested in a natural, physical result. So, for example, if a man is hungry, the Nefesh HaBahamis gives the man the drive to find food. If the needed food cannot be obtained, the signal from the Nefesh HaBahamis for food remains unabated. From the Nefesh HoElokis, the second soul, the neshomah, comes the brake on the Nefesh HaBahamis. It will signal a need to obtain the food in accordance with Torah. The Jekyl and Hyde syndrome comes from the knowledge that we has a Nefesh HaBahamis which has needs and a Nefesh HoElokis which seeks to control needs, or wants and to refine the Nefesh HaBahamis.
If our man is hungry and he cannot be satisfied with food because there is an obstacle in the way, the Nefesh HaBahamis activates the seichel (the intellect) to overcome that obstacle. The need to overcome the obstacle is
pareve. It is not good, not bad, simply necessary.
If the obstacle is great enough, the Nefesh HaBahamis will rise to the occasion finally doing whatever is required to succeed. If theft is necessary, it will steal; if murder is necessary, it will kill. If however the Nefesh HaBahamis resolves to steal, kill or do other wrong, the Nefesh HoElokis will engage in battle with it. The moment there is a resolution to transgress a mitzvah, there is a moral decision to be made. The Nefesh HoElokis will come into play and, depending on the relative strength of the person’s Nefesh HaBahamis, it will either win or lose the battle.
An important reality is that it will win or lose simply as a result of training. The Nefesh HoElokis does not succeed automatically. Victory is a matter of training in exactly the same way as the human body can increase muscle by exercise. Everything physical is an extension of what exists in the spiritual.
The fact that something exists in the physical world is only because it is a spiritual truth. In the same way, the Nefesh HoElokis with exercise becomes stronger and the Nefesh HaBahamis denied exercise atrophies. This is why of course it is so vitally important to educate children in Torah from the very earliest possible moment, so that their Nefesh HoElokis becomes as vigorous as possible and their Nefesh HaBahamis remains as innocent as possible.
Now, although the Nefesh HaBahamis is pareve, not bad, not good, the desire to overcome obstacles contrary to Torah is of course not good. Born from the Nefesh HaBahamis is the yetzer hora (Evil Inclination). It is a fundamental cornerstone of Torah that every Jew (apart from very special people discussed later) has a yetzer hora. The yetzer hora means literally an inclination for bad. What does “ bad ” mean? “ Bad ” means contrary to Torah and mitzvos. Every Jew has a yetzer hora according to the strength of his Nefesh HaBahamis. The Nefesh HaBahamis is strong in everybody because they are born with a Nefesh HaBahamis. The Nefesh HoElokis requires cultivation. The yetzer hora will be as strong as a man is at any point of time in his life. The Nefesh HoElokis of a Jew spawns the yetzer tov (Good Inclination), the desire to do good. What does “ good ” mean? “ Good ” means learning Torah and doing mitzvos. Every Jew has a yetzer tov, part of the Nefesh HoElokis.
When a Jew therefore is confronted with a life problem, there will be functioning equally at the same moment, his yetzer hora and his yetzer tov. The yetzer hora functions so as to achieve whatever the need is at any given moment; if there is an obstacle which requires doing bad to overcome it, bad will be done.
If the obstacle requires bad to be done inventively, it will be done inventively. But always the yetzer tov has the potential to overcome the yetzer hora and ultimately even to turn it to do good. Curiously, the Nefesh HoElokis has the capacity to reveal itself instantly. If a person’s teshuvah (return) is great enough, if a person’s need for Torah is strong enough, we are promised that the yetzer tov can be called upon completely. When this takes place, it becomes the total and effective brake on the yetzer hora resulting in real free choice in a Jew making moral decisions. Real free choice exists when the yetzer tov is as strong as the yetzer hora. There is the capacity in every Jew to succeed in overcoming his yetzer hora on all occasions and without fail.
Meanwhile, every man is tested until the day he dies; the more sophisticated he is, the more sophisticated will be his yetzer hora. The more attention therefore must be given in cultivating his yetzer tov.
At a deeper level the yetzer hora is not an evil force in itself. Since everything created by Hashem is for good, ultimately the yetzer hora must be for good. It is for good because the point of the yetzer hora is to allow man to exercise free choice in order to grow and blossom.
There was once a King whose only son was born with a birthmark. The King sent him, incognito, to the colonies. Before he left, the King cautioned his son on no account to gamble while away. The Prince swore to his father never to accept any kind of wager.
Successful in the colonies, the son ultimately made his way home, passing through a problem province. The Mayor of the province advocated a course of action contrary to the King’s interest and so of course the Prince objected. The Prince and Mayor argued until at last the Prince who could withhold his patience no longer revealed his identity and required obedience from the Mayor. The Mayor challenged his authenticity, wagering one thousand rubles that the young man was not the Prince. One thousand rubles! The Prince remembered his promise to his father but this, after all, was certain. He accepted the bet, showed his birthmark and, of course, collected the thousand rubles.
When the Prince arrived home, he found the King devastated.
“ How could you do it? ”, asked the King. “ You swore to me you wouldn’t take a bet. You swore. ”
“ This is different. ” replied the Prince. “ First of all, the disagreement related to your sovereignty. Secondly, I couldn’t lose the bet. ”
“ Fool. ” replied the King. “ I bet the Mayor a million rubles you wouldn’t accept. ”
The King is Hashem and, of course, the son is Am Yisrael. The Mayor is the yetzer hora. The yetzer hora is the tester of whether one triumphs in his potential for growth or the reverse. It is critical to see that although the Mayor has the short term reward of winning the bet, his love for the King makes him disappointed at the failure of his Prince. Always the rewards are greater for the triumph of the yetzer tov although not always immediately obvious.