"For this thing is very near to you, in your mouth and in your heart, so you can fulfill it" (Deuteronomy 30:14). The Tanya's seventeenth chapter takes this verse—which seems to promise that serving God is easy—and asks the obvious question: is it actually easy? Because it does not feel easy at all.
Changing your heart is not easy. The Talmud itself asks: "Is fear of Heaven a small thing?" (Berachot 33b). If even fear of God is hard, how much harder is love? And the rabbis explicitly say that "only tzaddik (a righteous person)im (the righteous) have control over their hearts." For everyone else, the heart does what it wants.
Rabbi Schneur Zalman resolves the contradiction. The verse is not talking about the kind of love that "glows like burning coals"—the ecstatic, overwhelming passion of the tzaddik. It is talking about a more modest but equally valid love: the love that leads to action. The re'uta d'liba (רעותא דלבא), the "hidden desire of the heart"—the quiet, almost imperceptible yearning for God that exists in every Jewish soul—is sufficient to drive a person to fulfill the commandments.
And this love is indeed "very near." Why? Because the mind controls the body. The Tanya makes a physiological claim: the brain has inherent authority over the heart, the mouth, and all the limbs. A person can always choose to speak words of Torah, to perform a mitzvah, to redirect thought toward God. The heart may resist, but the mind can override it.
The one exception: the person who is truly, completely wicked—the rasha gamur—whose repeated sins have so thickened the barrier between his soul and God that the mind has lost its authority over the heart entirely. The Torah "does not speak of the dead" (Berachot 18b), the Tanya says, meaning that the completely wicked are spiritually dead in the sense that normal spiritual mechanics no longer apply to them. But for everyone else—for the vast majority of humanity—the mind can always command the body. And that is enough.