(Devarim 11:18) "And you shall place these words upon your hearts and upon your souls": Scripture hereby apprises us that words of Torah are comparable to a drug of life. An analogy: A father strikes his son a stunning blow and places a plaster on his wound, telling him: "My son, so long as this plaster is on your wound, you may eat and drink as you wish and bathe both in hot and cold and no harm will come to you.
But if you remove it, you will raise an infection." Likewise, the Holy One Blessed be He says to Israel: My children, I have created in you a yetzer hara (an evil inclination), and I have created Torah as its antidote. So long as you occupy yourselves with it, it will not prevail over you, as it is written (Bereshith 4:7) "If you do 'good' (i.e., if you occupy yourself with 'good' = Torah), it (the yetzer hara) will be 'lifted' from you."
But if you do not occupy yourselves with Torah, you will be delivered into its hand, viz. (Ibid.) "But if you do not do 'good,' sin crouches at the door." And, what is more, all of its concourse is with you, viz. (Ibid.) "and to you is its desire." And if you so will, you can prevail over it, as it is written (Ibid.) "and you can rule over it."
And it is written (Proverbs 25:21-22) "If your foe is hungry, feed him bread," the "bread" of Torah; "and if he is hungry give him water (= Torah) to drink. For you thereby scoop out coals upon its head," (suffocating the "great flame"). "evil" is the yetzer hara (the evil inclination). Its Creator testifies about it that it is evil, viz. (Bereshith 8:21) "for the yetzer of man's heart is evil from his youth."