Tefilin

(9) “And this shall serve you as a sign on your hand and as a reminder on your forehead—in order that the Teaching of the LORD may be in your mouth—that with a mighty hand the LORD freed you from Egypt.

(5) When putting them on, have in mind that God commanded us to put these four passages which contain [the principle of] monotheism and the Exodus on the arm opposite the heart and the head opposite the brain so that we may remember the miracles and wonders that He did for us which indicate His Unity and that He is omnipotent in heaven and on earth to do with them as He wills. And to submit to God his soul, which resides in the brain as well as his heart which represents the main physical desires and thoughts.

The verse says, "Where shall I find wisdom," teaches us that King Solomon was searching for the source of wisdom. R' Eliezer teaches that it's found in the head. R' Yehoshua teaches that it's found in the heart.

It is known that true beliefs such as: belief in the creation of the world, G-d's providence and that G-d resides amongst the Jewish nation only expresses itself through physical actions. Through these physical actions, the knowledge establishes itself in ones heart. The knowledge of creation of the world is effected by the act of keeping Shabbos, the knowledge of divine providence through the action of mezuzah and that the Shechina (divine presence) rests among the Jewish people is expressed in the mitzvah of tefillin. In order that this philosphy becomes imprinted on our hearts we're commanded to place the tefillin on the two parts of the body that are the source of all thoughts and actions: the mind and the heart.

(1) 'מצרף לכסף וכור לזהב ובוחן לבות יהוה, “the crucible is for silver, the furnace for gold, but G’d examines the hearts.” (Proverbs 17,3)(2) In this verse Solomon warns man to refine his thoughts and to improve his character traits. Seeing that the heart is the most important one of our organs and all other organs depend on it and it is situated at the center of the body, it governs the other parts of the body, guides them and inspires them with its energy. All other organs of the body are subservient to the heart. Symbolically speaking, the heart is comparable to the king of a country who resides in its center, supervises all the more peripheral parts of the country, and who issues decrees telling his subjects how to conduct themselves.
Seeing that all other organs depend on the heart for their continued existence, G’d, who is the source of life to all His creatures is also referred to as “heart.” This is the meaning of Psalms 73,26: “the Rock of my heart and the Lord is forever my portion.” Just as the human heart exercises its power and influence on other organs of the body so G’d exerts influence on all parts of His universe to the farthest corners of it. This is confirmed in Exodus 8,18: “for I the Lord am active in the midst of the earth.”
Similarly, the most important element in relating to Torah and how one observes it is the heart. The entire written Torah is bracketed between the letters of the word לב, heart, i.e. between the letter in ב-ראשית at the beginning and the letter ל in ישרא-ל at the end. This symbolizes that the Torah itself was taken from the attribute (emanation) חכמה which has לב, 32 paths...

AND ESAU SAID IN HIS HEART. Rabbi Abraham ibn Ezra said, “It is possible that he revealed his secret to one of his friends.”
But this is not necessarily so for any decision which a person reaches after deliberation is referred to in Hebrew as “speaking with the heart,” even when it also includes speech with the lips. A similar case is found in the verse, And with a double heart do they speak, for the will is synonymous with the heart. And all Israel were of one heart to make David king, meaning that they had all arrived at the same decision and were talking of it.

והביא את אשמו ליהוה על חטאתו אשר חטא, נקבה מן הצאן, “he shall bring as his guilt-offering to Hashem a female from the flock, etc.” Our sages called this paragraph the עולה ויורד, meaning that although the expression אשמו occurs here the nature of the offering is not the kind of offering called אשם elsewhere at all... Why then do we encounter the word אשמו usually translated as “his guilt-offering?” The reason is that his sin or guilt is the cause of his having to bring an offering. Just as on occasion a verse in Scripture refers to חכמה, “wisdom” as לב, “heart,” seeing that the heart is the seat or cause of wisdom, so here the Torah refers to the cause for the offering and calls it by that name.

Tefillin: However the content of tefillin is much greater than tsitsit. And that is that the Creator, may He be blessed, granted that Israel truly bring down His holiness, may He be blessed; and that they be crowned by it in such a way that all of their spiritual and physical characteristics be sheltered under this great light; and that they [also] create a great refinement through it. And this is what the verse states (Deuteronomy 28:10), "And all the nations of the world will see that the name of the Lord is called upon you." And this matter is dependent upon this commandment, in all of its laws and details. Moreover, there are two main organs in a person from which the soul derives great strength. And they are the brain and the heart. And the Creator, may He be blessed, commanded that this light first be brought down into the brain through the head tefillin and that the brain, and the soul in it, be refined through it. Then it expands afterwards to the heart through the arm tefillin that is across from it, so that it too is refined by it. And through this, it comes out that man is completely included in the drawing down of this holiness to all of his characteristics; he is crowned by it and attains a great holiness. Notwithstanding, there are various details found in the conditions of the commandment: Matters are found in all of their parts that are needed for the desired refinement of all of his parts, according to the division of the characteristics of man.