In Parshat Bereishit, we are introduced to the real battle that each individual must fight throughout his life. This is the battle with the Yeitzer Hara, the evil inclination.
Hashem explains this to Cain, after he kills his brother. He tells him that if he chooses good, he will carry it, the Yeitzer. And if the Yeitzer wins, he will himself at the doorway of sin.
At the end of the Parsha, Hashem makes the realization that the inclination of man, is only evil, from his youth. It is now clear that overcoming the Yeitzer Hara, is so challenging, that it cannot be accomplished without laws, to help win this war.
Clearly defined rules, with clearly defined consequences, are the only way not to succumb, to the temptations of the Yeitzer. Without such clarity, man will rationalize and fall.
There is a story in one of the Mussar books, of Jews that come back victorious after fighting a war. When they report this victory to their sage, he tells that the real war has just begun.
There must be a daily awareness that decisions will need to be made. The Yeitzer Hara’s job, is to fill us with doubts, and entice us into doing the wrong thing. Such an awareness will motivate us, to overcome such temptations.
The Talmud in Kiddushin, says that Torah is the antidote to the Yeitzer Hara. If you want to weaken it, drag it into a house of study. Cleaving to the principles of the Torah, will allow us to remain strong and victorious over the Yeitzer Hara.