“This day G-d commands you to fulfill [all the laws], and you should do them with all your heart, and with all your soul.” (26:16)
The Midrash Tanaim jumps on the classic interpretation (see also Tanchuma 1) that when Moshe says “This day” with reference to mitzvah observance, he is noting that any demonstration of following the Torah creates a visual for G-d as if the Jewish people have accepted the Torah on that day.
It is possible that “L’vavkha” (your heart) is plural, suggesting more than one heart. Obviously every individual only has one heart. Midrash Tanaim suggests that the soul is also considered the heart. We know that a body without a heart cannot live. Perhaps the homiletic jump is not that far off, that a soulless individual has no real life.
On the other hand, the Rosh argues that when a person prays he should be careful not to have two hearts, one focused on G-d and the other focused on other things.
Rabbenu Bachaye looks at the verse we’re discussing and addresses the difference between this and a similar sentiment in the Shema. Moshe told us there, “And you shall love G-d with all your heart, all your soul, and all your essence … these words that I command you today should be on your heart.” The message there was also, as the Rabbis expounded, that the words of the Torah being commanded “today” should be new in your eyes always. After all, generations change, and the heart of Man is drawn after what he sees.
It is very easy for those who did not see or experience Revelation to have a difficult time relating to it. Yet something has to serve as the anchor that holds our people together.
This may be why the Shema is exactly that — a call to “Hear” and not to see. Seeing, after all, is a better form of proof leading to belief than merely hearing something second hand. For any event in history, only a few people are privy to actually witnessing it — but if after the fact everyone talks about it, allowing others to “hear” about it, then word can spread forever.