The loah mahfuth or the guarded tablet is the heart of the Prophet Muhammad (pbuh), which is most clearly seen in Surah 26 or the Poets (Al Shu'ara). This Surah first mentions the stories of Bani Israel and they are the story of Musa, Ibrahim,and Nuh. The stories of the Prophets unknown to Bani Israel are next addressed and they are Salih, Hud and Shuaib with Lut following Salih and preceeding Shuaib. (This means that Lut was sent to a people who inhabited the territories between Mada'in Salih and Midian in the Northwest of the Peninsula). So, since this Surah is addressing the Jewish tribes in Arabia, one must refer to their scriptures to find the image invoked by the words loah mahfuth. Let's begin by referencing the ayas which explain that the revelation was impressed upon the heart (meaning the unsullied understanding of the pure mind) of the Prophet (pbuh) and notice the specific reference to Bani-Israel:
192 Truly this is a revelation from the Lord of the universe:
193 With it came down the spirit of faith and truth,
194 To your heart and mind, that you may admonish,
195 In the perspicuous Arabic tongue.
196 Without doubt it is in the mystic books of former peoples.
197 Is it not a sign to them that the learned of Bani-Israel knew it?
Since Allah (swt) is specifically appealing to the Jewish tribes in Arabia in this Surah, He uses the imagery of their scriptures. The comparison of the heart to an inscribed tablet can be found in the following Biblical verse:
Jerimiah 17:1: The sin of Judah [is] written with a pen of iron, [and] with the point of a diamond: [it is] graven upon the tablet of their heart..
The Hebrew words for tablet of their heart is loah (loah=tablet) libam (lib=heart).
The Christian tribes in Arabia would also be familiar with the comparison of the heart to an inscribed tablet as the Epistles ascribed to Paul also use the same image:
2Corinthians 3:3: And you show that you are a letter from Christ delivered by us, written not with ink but with the Spirit of the living God, not on tablets of stone but on tablets of human hearts.
In conclusion, the loah mahfuth or the guarded tablet refers to the understanding 'heart' of the Prophet whose purity Allah (swt) guarded so that the revelation would also remain pure of imperfections and contradictions. The Jewish and Christian tribes in Arabia would have especially understood this reference as it was used in both of their scriptures. There was no Arabic translation of any of these scriptures, which were at the time only available in Latin, Greek and Coptic. Since the Prophet (pbuh) had no access to these scriptures, was illiterate, and did not know the foreign languages in which these scriptures were written, the use of this image is another proof of the miraculous revelation of the Quran.
And Allah (swt) knows best!