"Know this day and take it to your heart that Hashem is G-d; in the heavens above and upon the earth below there is nothing else."[i]
Clearly, we are being commanded to take this knowledge of Hashem being all into our heart. The heart is the seat of thoughts and emotions. So this means we are to contemplate or meditate on this knowledge in order to bring it into our thoughts and feelings.
But we are also told to fulfill Hashem's commandments and to "not follow your heart and after your eyes by which you go astray…."[ii]
So which is it? Are we to rely on our hearts or not? And if we are already told to know that Hashem is all ("Know this day"), why do we need to take it to our hearts? We already know it. Isn't that enough? And if we are commanded to take this holy knowledge to heart, why are we also told not to follow our hearts?
"Knowing it," is getting it. You understand it clearly in your head and you will act accordingly. And certainly the knowing intellect is higher than the emotional heart. But the heart can add something that the intellect does not insist upon... the love, the warmth, and the excitement of knowing these holy things. These emotions come from the heart.
Then why are we told not to follow our hearts and eyes because they will lead us astray? The heart and eyes without the intellect can quickly fall to the entirely animal instinct. The animal seeks instant gratification, not such lofty things as spiritual awareness and fulfilling Hashem's commandments. So we are warned not to follow our hearts and eyes. But when the Intellect, the knowledge, is present then indeed we do want the emotional aspects of service to Hashem to be brought into our lives.
Like almost everything in this amazing creation, the emotional heart can be used for good or for bad. The intellect must tell us what to love, but the heart will tell us how to love it.