You shall bind them as a sign upon your hand, and they shall be for a reminder between your eyes.
-- Deuteronomy 6:8
You have a brain. It is in one world. Your heart is in another. And your hands often end up involved in something completely foreign to both of them. Three diverse machines.
So you put on tefillin. First thing in the day, you connect your head, your heart and your hand with these leather cables—all to work as one, with one intent. And then, when you go out to meet the world, all your actions find harmony in a single coordinated purpose . . .
Tefillin are a pair of black leather boxes containing Hebrew parchment scrolls. A set includes two tefillin—one for the head and one for the arm. Each consists of three main components: the scrolls, the box and the strap. The scrolls are inserted into boxes made of leather that have been painted black.
One box is strapped on your head, and the other onto your arm next to your heart. It’s done once a day—preferably during the morning prayers—while you say a passage called the Shema Yisrael. It’s done by Jewish males, age 13 and up, every day except Shabbat and major Jewish holidays.