הֲלוֹא אִם-תֵּיטִיב שְׂאֵּת וְׂאִם לֹא תֵּיטִיב לַפֶּתַח חַטָּאת רֹבֵּץ וְׂאֵּלֶּיךָ תְׂשׁוּקָּתוֹ וְׂאַתָּה תִמְׂשָּׁל-בּוֹ (ברשית ד,ז)
If you do not improve yourself, then sin is crouching at the entrance, longing to entice you; but you can rule over it.
Berachos 61a: Rav said: The evil inclination resembles a “fly” and sits between the two gateways of the heart, as it says, “Flies of death corrupt and putrefy perfumed oil” (Koheles 10,1). Shmuel said that it is like wheat (חטה) as it says, “Sin crouches at the door.”
Targum and Rashi explaining Rav: The evil inclination is like a fly that a single dying fly can fall into a large container of perfumed oil and make it putrid, so too the evil inclination can worm its way into the heart of someone whose reputation and good name are as fine and widely known as the fragrance of perfumed oil, and bring him to ruin.
Rashi explaining Shmuel: The term for sin (חטאת) is assonant with the Hebrew term for wheat (חטה).
Maharal explains the dispute between Rav and Shmuel: Rav maintains the evil inclination, which is part of man’s nature that inclines to evil, is itself degenerate and corrupt and is attracted to such things. He therefore likens it to a fly which is foul and attracted to that which is spoiled (corruption). Shmuel maintains that the evil inclination is what is most fine in a person, which perversely is attracted to evil and corruption, just like opposite poles attract. He therefore likens it to wheat which is especially choice and fine.
Torah Temimah: It appears to me that Rav and Shmuel do not argue. Shmuel by connecting to חטה is referencing the concept that the food item which Adam haRishon eat (that he should not have) was wheat, and as a result of the wheat being eaten, the evil inclination spread out through the world. Both Rav and Shmuel agree that initially as a result of the wheat being eaten the evil inclination strives to draw a person to the wrong path. However, if the person does not succumb to its influences the evil inclination adjusts and nibbles at the person’s feelings and emotions and this activity is similar to that of a fly.
The concept that the evil inclination sits between the two gateways of the heart has a halachik outcome. At several times one the custom is for one to strike one’s chest (during tachanun, Vidui on Yom Kippur, etc.) The Medrash Koheles says that one strikes towards the heart to indicate that the heart is the source for my problems. As a result of this Gemara, we understand that striking the “heart” does not mean literally the place of the heart, but means the breast above the heart, which is the abode of the evil inclination.
Gra: At the door opening sin crouches - The simplest toll the evil inclination uses in convincing a person to sin is by fooling him into thinking that this sin is not really a sin. Translate "la'pesach chatos roveitz" as "sin crouches at the door of permissiveness," a "pesach hetter." (there is already a breach in the person's mind). And this conforms perfectly with Chazal, who say in Sukah (26a) 'It is the breach that beckons to the thief'. What they mean is that the Yeitzer ha'Ra only has access to a person there where the person himself is in doubt whether to sin or not, or as long as he is in a quandary whether to perform a Mitzvah or not. That is the door that invites the Yeitzer ha'Ra in.
And this also explains, the Gra adds, why G-d said to Kayin "Why are you angry?" Kayin believed that having sinned, there was nothing he could do to keep the Yeitzer ha'Ra out. So G-d explained to him that just as it is initially possible to hold him at bay by keeping the door shut, so too, it is possible to prevent him from exploiting the situation, at this late stage, by expelling him firmly and by shutting the door before he enters has a chance to enter a second time. It may well be a more difficult task to accomplish, but it lies within one's power to do so. In other words, Kayin needed to strengthen his own convictions, dispelling all doubts from his mind, and the Yeitzer ha'Ra would be powerless to start with him again. "The Yeitzer ha'Ra crouches at your door," G-d told Kayin, "and longs for you". There is nothing he can do but long for a person to open the door. In the end, the Pasuk concludes, '"you can control him". Just keep the door shut (strengthen your own conviction of what is right), and he will remain outside, longing to gain entry, but incapable of doing so.
