Halakhah for Sanhedrin 42:4

That is that He commanded us that every man among us should write a Torah scroll for himself. And if he writes it with his hand, Scripture accounts it to him as if he received it at Mount Sinai. But if it is impossible for him to write it, he should buy it or hire someone to write it for him. And this is His saying, "write for yourselves this song " (Deuteronomy 31:19). And it is not permitted to write [only] sections. For when He said, "this song," He intended the whole Torah, which includes this song (of Haazinu). And the language of the Gemara (Sanhedrin 21b) is, "Rabbah said, 'Even if his ancestors left him a Torah scroll, it is a commandment to write a scroll of his own, as it is stated, "write for yourselves."' Abaye raised an objection to him (from a baraita concerning the king’s Torah scroll), '"And he writes himself a Torah scroll for himself, so that he does not become proud from the Torah scroll of his ancestors." A king, yes, but an ordinary person, no!'" And the answer was, "No, [the ruling of that baraita] was only necessary [to teach that the king is commanded to write] two Torahs; as it is taught in a baraita, 'And he must write two Torahs for himself.'" This means to say that the difference between a king and an ordinary person is that every man is obligated to write a Torah scroll, whereas a king, two - as it is explained in the second chapter of Sanhedrin. And the regulations of this commandment - meaning to say, the writing of a Torah scroll and its stipulations - have already been explained in the third chapter of Menachot and in Shabbat, Chapter 16. (See Parashat Nitzavim; Mishneh Torah, Tefillin, Mezuzah and the Torah Scroll.)

It is from the roots of the commandment [that it is] because the king is under his own power - no man will pain him over his actions nor rebuke him, and with the staff of his mouth he can plague his land and with the breath of his lips he can kill anyone he wants from his entire people. Therefore he truly needs great guarding, and a proper reminder standing in front of him [so that] he will always look upon it in order to conquer his impulse and turn his heart towards his Creator. And this is [the meaning of] that which they, may their memory be blessed, said (Sanhedrin 21b), "When he goes out to war, the Torah scroll is with him; sitting in judgement, it is with him; adjourns to eat, it is in front of him." The general rule of the matter is that it not move from in front of his eyes except when he needs to [relieve himself] or enters the bathhouse.

And Ramban, may his memory be blessed, answered the challenges and clarified the matter clearly, completely and nicely. And this is his language that he wrote in his commentary of the Torah: That matter which Rambam, may his memory be blessed, wrote that the commandments have explanations is something very elucidated, as in each one there is an explanation and a benefit and a refinement for a person, besides the reward for them from the Commander, may He be blessed. And they, may their memory be blessed, have already said (Sanhedrin 21b), "For why did He not reveal the explanation of the commandments, etc.?" And they, may their memory be blessed, expounded (Pesachim 119a), "'And to the clothed elegantly' (which can also be read as, 'and to the one that covers the ancient' Isaiah 23:18) - this is [referring] to the one who covers that which the Ancient of Days (God) covered. And what are they? The explanations of the Torah." And they have already expounded about the red heifer (Bemidbar Rabbah 19:6) that Shlomo said, "I have discerned everything, but the section of the red heifer, I have investigated and asked and searched - 'I said that I could fathom it, but is far from me' (Ecclesiastes 7:23)." And Rabbi Yossi Bar Chinanah said, "The Holy One, blessed be He, said to Moshe, 'Go and I will reveal to you the explanation of the heifer, but to another, it will be a statute (that is not understood),' as it is written (Zechariah 14:6), 'In that day, there shall be neither sunlight nor cold moonlight' - things that are covered from you in this world, in the future you will gaze upon them in the world to come; like that blind man that becomes able to gaze, as it is written (Isaiah 42:16), 'I will lead the blind by a road they did not know.' And it is written (later in the same verse), 'I have done these words and I have not left them' - as I have already done them for Rabbi Akiva," meaning to say that Rabbi Akiva already knew them in this world.