NATURAL IMPULSE
THE good impulse (yetser tov) and the evil impulse (yetser ra) are pictured in Jewish literature as wrestling in perpetual conflict within the heart of man. Satan is usually identified with the yetser ha-ra, the evil impulse. In the book of Job, Satan’s function is described as that of testing the sincerity of men’s characters. In Talmudic literature, Satan’s function is to strengthen man’s moral sense by leading him into temptation. It has been said that every man living shall assuredly meet with an hour of temptation, a certain critical hour, which shall more especially try his mettle.
According to a midrashic statement (Genesis Rabbah 9:9), the existence of the yetser ha-ra in the heart of man and the struggle to overcome it lends high value to the good that emerges from the inner battle. The two conflicting impulses, the good and bad tendencies, are said to be implanted in man as a consequence of his having been formed from the dust and endowed with a soul (Genesis 2:7).
According to rabbinic thinking, the evil impulse is to be found in man at birth; the good impulse begins to develop when he is thirteen years old. The teachings of the Torah are referred to as the antidote to the yetser ha-ra. Similarly, Ben Sira (21:11) states: “The man who keeps the Law controls his natural tendency.”
In commenting on the two yods in the word “omitted Hebrew character here”, (Genesis 2:7), the rabbis declare that God created both the yetser tov and the yetser ra (Berakhoth 61a). The command to love God “with all your heart” they interpret to mean “with both your impulses” (Berakhoth Ma), since both human elements can be employed in the service of God. “Were it not for the yetser ha-ra, no man would build a home or get married or follow an occupation” (Genesis Rabbah 9:9). The phrase “very good” (Genesis 1:31) is therefore explained, as alluding to the yetser ha-ra, frequently used in the sense of the productive urge.
Taken from the Encyclopedia of Jewish Concepts
Genesis 1:31
And God saw everything that he had made, and behold, it was very good. And there was evening and there was morning, a sixth day.
Genesis 6:1-6
1 When human beings began to grow numerous on the earth and daughters were born to them,
2 the sons of God saw how beautiful the daughters of human beings were, and so they took for their wives whomever they pleased.
3 Then the LORD said: My spirit shall not remain in human beings forever, because they are only flesh. Their days shall comprise one hundred and twenty years.
4 The Nephilim appeared on earth in those days, as well as later, after the sons of God had intercourse with the daughters of human beings, who bore them sons. They were the heroes of old, the men of renown.
Warning of the Flood.
5 When the LORD saw how great the wickedness of human beings was on earth, and how every desire that their heart conceived was always nothing but evil,
6 the LORD regretted making human beings on the earth, and his heart was grieved.