The Parsha begins with the words: אִם־בְּחֻקֹּתַי תֵּלֵּכוּ גּוֹ׳ “If you follow My statutes etc”. The Alter Rebbe explains in Likkutei Torah (at the beginning of our Parsha the Chassidishe Parsha, which was the second Parsha for this week), as well as in the Maamorim of the Chabad Rebbeim in the generations that followed. The word ‘BeChukoisai’ is an expression of ‘Chakikah’ which means to engrave. Because the highest form of the Torah is in a manner of the ‘Engraved letters’. As the Possuk states: חָרוּת עַל־הַלֻחֹּת “Engraved on the Luchos”.
The next form of Torah תוֹרָה שֶׁבִכְּתָב is condensed into the ‘Written letter’. And thereafter it descends further to the next level down which is the ‘Oral Torah’.
The difference between them is that the engraved letters are engraved into the stone, so that the letters are integral to the stone upon which they are engraved making it is impossible to separate or to differentiate between them.
This is the concept of engraved letters in Torah. They are Mamosh one with Hakodosh Boruch Hu. Below this level the Torah is contracted into the written letter. As is explained in Tanya that the Torah travelled and descended until it took on the form of ink on a book where the ink and the book are two independent elements that can be separated.
There are two perspectives of this united entity of ‘engraved letters’ and the inseparable stone upon which they are written: The first is that the letters cannot be separated because they are an integral part of the stone. The second aspect is, that you cannot separate the stone from the letters, so the distinctiveness of these stones is that they have the letters engraved upon them.
