Halakhah sa Exodo 10:1: Sefer HaChinukh

The commandment of the tefillin of the head: To place tefillin on the head, as it is stated (Deuteronomy 6:8), “and they shall be as totafot between your eyes.” Behold, I wrote in the previous commandment what is the content of the tefillin, that it is four sections that are written in the Torah, in the Order of Bo el Pharaoh, in the Order of Va’etchanan and in the Order of Vehaya Ekev. And we were commanded to write these four sections on parchment and to place them on our heads between our eyes and on our upper arm across from the heart. And the matter of these four sections more than other sections of the Torah is because there is in these the acceptance of the yoke of Heaven, the unification of God, and the matter of the exodus from Egypt which forces belief in the creation of the world and the supervision of God over the lower beings — and these are the fundamentals of the Jewish religion. And therefore we were commanded to place these fundamentals between our eyes and upon the tablet of our hearts all day; as the wise men of science have said that these two limbs are the residence of the intellect. And when we place these things upon them as a memory device, we are strengthened about them, and we add to the cognizance of the ways of God, may He be blessed; and [so] we merit life in the world to come. And some of the laws of the tefillin are written above in that of the arm.