קבלה על שמות 13:2

The first words that appear in the Tefilin read, "Sanctify to Me all the firstborn (Heb. קַדֶּשׁ לִי...)" (Exodus 13:2). This corresponds to Yud, which is holiness - which is the firstborn of all the supernal sanctities. These are "whatever opens the womb" (Exodus 13:12) by that narrow line that descends from the Yud, which opens the womb to bring forth proper fruit and seeds. This is the supernal holiness.