Quoting the source of the issur of hesech ha’da’as from the gemara. A machlokes Rishonim as to the definition of hesech ha’da’as. A contradiction in the Rambam, A second contradiction in the Tur. A new yesod in the din of hesech ha’da’as. Answering the Rambam and the Tur. A strong ruchniyus message, bringing out the goal of mitzvas tefillin – bringing the person to dveikes in Hashem.