Shulchan Aruch, Din Kesivas HaTefilin 32:64a
Rabbi Avraham Meyer Zajac – Within decribing one of the halachas (laws) of tefillin, the Rabbi shares with us about both letter Shins on the head tefillin and their deeper meanings as it relates to Kabalah and Chassidus – one representing the essence of G-dliness that never changes and one representing the revelation of G-dliness that constantly changes. Deep stuff! Hope you enjoy it.