The injunction of the Shema that the Israelites are to place the words of God upon their heart, hand, forehead, and doorposts (Deut. 6:6–9), clearly a metaphorical demand for constant meditation on God’s commandments, was, by the second century BCE, interpreted literally, giving rise to the custom of wearing tefillin, phylacteries (Matt. 23:5), and affixing mezuzot to the doorposts.
