Rabbi Yaakov Weinberg zt”l: Notes on Devarim I
The entire Torah was said by Moshe to all of the Jews, So what does the pasuk mean that These are the words that Moshe told Klal Yisrael’? It implies that there were some parts that Moshe told Aharon or other talmidim and they told Bnai Yisrael-which is not true.
The Rosh Yeshiva ZTL explained that the pshat is that the rest of Torah is G-d’s words directly. All conversations between people in the Torah became Torah only when Kavayachol told Moshe these conversations later at Sinai.
When Phaaroh spoke to Avraham, his words were not Torah. They became Torah when Hashem said it to Moshe. Devorim works this way as well.
Moshe had deep and profound thoughts which he shared with Klal Yisrael and later Hashem decided to make it part of Torah. The rest of Torah was made when Moshe said something to Bnai Yisrael- it was directly Torah.
Moshe was a direct shiiach Hashem. In Devorim, Moshe’s words only became Torah after Hashem told Moshe that they will part of Torah. So in Devorim, we must think on 2 levels. #1, what was Moshe, the greatest of all human beings, thinking when he told Bnai Yisrael these things.
#2, what are the eternal absolute values that we learn now that Hashem has made Moshe’s words part of His Chumash. [One might say then that Sefer Devorim requires more iyun and not less (as a result of it being Moshe’s words (and ‘not directly Hashem’s’) then the other Seforim of Chumash.-ed.]
This applies in terms of mitzvos as well. The fact that Hashem chose to describe and mention certain mitzvos mentioned in Sefer Devorim as if Moshe is the one revealing them without a classic ‘Vayedaber Hashem el Moshe. ..’ is very instructive to us.
All of the mitzvos that are mentioned only in Sefer Devorim (and the ones that are repeated) have a human element to them that Hashem wanted to express through the fact
that Moshe mentioned them in his speeches.
Afterwards, Hashem decided to write them in the Chumash in this forum only(and not mention it in any of the other Seforim). For example, the emotional connection that
all Jews have to the Shma (expressed by our reciting it at death, etc.) may not have existed if the Torah hadn’t given that mitzvah without Moshe’s human element in Devorim. This approach applies to all mitzvos in Devorim except for where it is obvious that Hashem is talking and not Moshe.