In Hebrew a “lev tov,” good heart, is spelled as לב טוֹב. These letters numerically equal 49, a symbolic number that would represent the 49 levels of “tahara,” purity, that one can reach with a pure heart.
The gematria of תאוה (desire) equals that of בית (home). Indeed the yetzer hara, symbolic of desire, wishes to to dwell in the heart of man, but must be constantly evicted so that man’s heart can remain pure.
The Gemara (Brachos 61a) brings down that Rav says: The yetzer hara (evil inclination) is like a fly that sits on two openings of the heart, as the pasuk says: “Flies of death will spoil the perfumer’s oil” (Kohelet 10:1). In the book “Aleinu L’Shabei’ach” written by Rav Zilbertein and Rav Moshe Zoren, the authors explain that a fly has heightened seeing capabilities which is comparable to the yetzer hara that attempts to make us stray with our eyes. In fact, a fly has two compound eyes and each eye is made up between 3,000 and 6,000 simple eyes. We know the verse in Kriyat Shema says, “V’lo taturu acharei l’vavchem … “And do not explore after your heart and after your eyes after which you stray,” mandating us to be on high alert to guard our eyes with even greater fortitude.
The first letter of the Torah is ב (beis), as it begins בְּרֵאשִׁית. The last letter of the Torah is ל (lamed), as it ends ישראל. Therefore, frontwards, combining the beis and lamed the word emerges as בל “not to.” Starting from the end of the Torah backwards, the word spells לֵב “heart.” The message is that one must start the Torah with בל “not to,” keeping in mind the boundaries of morality; of what is permitted and prohibited. Only after mastering these aspects of obligatory behavior can one enter the realm of לֵב “heart,” to approach G-d with an all consuming heart of love.