“And you shall tie them on your arm as a sign” (Devarim 6,8)
It is a positive commandment to tie tefillin on the arm as it says “and you shall tie them as a sign on your hand” and the meaning of this verse is that we should tie the four chapters from the Torah on our arms, and they are called “tefillin” when they are tied with straps. And these are the four chapters: two of them are from the end of Parshas Bo, the chapter of “kadesh li kol bechor” and the second chapter is “vehaya ki yeviacha”. And the third chapter is from the end of Parshas Vaeschanan and that is the chapter of “Shema Yisrael” until “uvisharecha”. The fourth chapter is at the end of Parshas Ekev, and that is the chapter of “vehaya im shamoa tishmeun” until “kimei hashamyim al ha’aretz”.
We write these four chapters on one parchment, and rule it up like a Torah scroll from the end to the beginning, and place it in a box made from leather and pass one strap from the end of the leather and tie that piece of leather with the chapters inside it on the left forearm and after they are tied on the forearm they lie opposite the heart and they are the ones that are referred to in all places as “Tefillin shel Yad”.
Amongst the roots of the commandment, since a person is a physical entity he is forcefully drawn after physical desire; for the nature of physicality is to seek all pleasures and delights like a horse or mule with no understanding, if not for the fact that the soul which G-d favored him with restrains him, to the best of its capabilities, from sin, and since the soul resides with the body on earth, for the earth is the place of the body, and it is far away from heaven which is the place most fitting for it, the soul cannot easily overcome the body, but needs many guards to protect it. The Omnipresent One, blessed be He, wanted to merit us, we who are the holy people, and He commanded us to set up strong guards around the soul, and they are that we are commanded not to stop speaking words of Torah day and night, and that we should put four tzitzis on the four corners of our clothes, and a mezuza on our doorways, and tefillin on our arms and on our heads, and it is all to remind us so that we should cease any kind of theft and we should not be led astray after our eyes and after the willful thoughts of our hearts.
Amongst the laws of this commandment, that he must tie it on the bicep and that is the blown-up flesh between the shoulder joint and the elbow so that when he puts his elbow close to his ribs the tefillin will rest opposite his heart and he will thereby fulfil the commandment “and these words shall be on your heart”.
One who transgresses this and does not lay tefillin on the arm and on the head, is transgressing eight positive commandments, for the Torah has commanded us about four chapters in the tefillin of the arm and (four) of the head.