(ז) רבי נחמן בר שמואל בר נחמן בשם רב שמואל בר נחמן אמר: הנה טוב מאד, זה יצר טוב. והנה טוב מאד, זה יצר רע. וכי יצר הרע טוב מאד אתמהא?! אלא שאלולי יצר הרע, לא בנה אדם בית, ולא נשא אשה, ולא הוליד, ולא נשא ונתן. וכן שלמה אומר: (קהלת ד): כי היא קנאת איש מרעהו:
(7) Rabbi Nahman said in Rabbi Samuel's name: 'Behold, it was good' refers to the Good Desire; 'And behold, it was very good' refers to the Evil Desire. (It only says 'very good' after man was created with both the good and bad inclinations, in all other cases it only says 'and God saw that it was good') Can then the Evil Desire be very good? That would be extraordinary! But without the Evil Desire, however, no man would build a house, take a wife and beget children; and thus said Solomon: 'Again, I considered all labour and all excelling in work, that it is a man's rivalry with his neighbour.' (Ecclesiastes 4:4).
עבריתאותיבו בתעניתא תלתא יומין ותלתא לילואתא מסרוהו ניהליהו נפק אתא כי גוריא דנורא מבית קדשי הקדשים ... אמר להו חזו דאי קטליתו ליה לההוא כליא עלמא חבשוהו תלתא יומי ובעו ביעתא בת יומא בכל ארץ ישראל ולא אשתכח אמרי היכי נעביד נקטליה כליא עלמא ניבעי רחמי אפלגא פלגא ברקיעא לא יהבי כחלינהו לעיניה ושבקוהו ואהני דלא מיגרי ביה לאיניש בקריבתה
English
They (The Ancient Sages) ordered a fast of three days and three nights, whereupon he (The Yetzer HaRa) was surrendered to them. He came forth from the Holy of Holies like a young fiery lion. He (one of the rabbis) said to them: Realize that if you kill him, the world goes down. They imprisoned him for three days, then looked in the whole land of Israel for a fresh egg and could not find it. Thereupon they said: What shall we do now? Shall we kill him? The world would then go down. Shall we beg for half-mercy? They do not grant ‘halves’ in heaven. They put out his eyes and let him go. It helped inasmuch as he no more entices men to commit incest.
The yetzer hara is 13 years older than the yetzer hatov. While still in the mother's womb, the yetzer hara begins to develop in a person. If he begins to violate the Sabbath, nothing stops him. If he commits murder, nothing stops him. If he goes off to another sin, nothing stops him.
But 13 years later, the yetzer hatov is born. When he violates the Sabbath, it rebukes him, "Airhead [literally: "empty one"]! Don't you know it says 'Everyone who violates it will surely be put to death' (Exodus 31:14)?" If he is about to commit murder, it rebukes him, "Airhead! Don't you know it says 'Whoever sheds a man's blood, by man will his blood be shed' (Genesis 9:6)?" If he is about to engage in a sexual sin, it rebukes him, "Airhead! Don't you know it says 'Both the adulterer and the adulteress will surely be put to death' (Leviticus 20:10)?"(Avot d'Rabbi Natan 16).