In the beginning of Nitzavim:
Who are these who are not present today? I could suggest that obviously not all of klal yisrael was present before Moshe, because they would not all fit, and that instead we have a representative sample, which deliberately included segments from the entire Israelite population, men, women and children, from the most important judges to the least important water carriers. But these are representative, so not just those who are physically present on the day of the covenant, standing before Moshe, but also those at home whom they represent.
However, it also makes good peshat sense to understand it as the future generations. We have a few pesukim later:
|כא וְאָמַר הַדּוֹר הָאַחֲרוֹן, בְּנֵיכֶם אֲשֶׁר יָקוּמוּ מֵאַחֲרֵיכֶם, וְהַנָּכְרִי, אֲשֶׁר יָבֹא מֵאֶרֶץ רְחוֹקָה; וְרָאוּ אֶת-מַכּוֹת הָאָרֶץ הַהִוא, וְאֶת-תַּחֲלֻאֶיהָ, אֲשֶׁר-חִלָּה יְהוָה, בָּהּ.||21 And the generation to come, your children that shall rise up after you, and the foreigner that shall come from a far land, shall say, when they see the plagues of that land, and the sicknesses wherewith the LORD hath made it sick;|
Even so, I don't find this entirely persuasive, and like my own suggestion. (The flaw in my suggestion according to many, would likely be that this is coming to be all-encompassing, of every Israelite, so there would be no living Israelite who was not present.) Regardless, the idea that it refers to subsequent generations is a popular and traditional one. And Ibn Ezra says it:
[כט, יג]ולא אתכם לבדכם -רק עמכם ועם הבאים אחריכם, שהם בניכם ובני בניכם.[כט, יד]נו"ן ישנו. נוסף.ואת אשר איננו -עם אשר איננו, רק יבא אחרינו ואינו כטעם האומרים, כי רוחות הבאים היו שם.
Thus, these are those who come after, who are your sons and grandsons. And here is the interesting part. Ibn Ezra rejects those who say that the spirits who came were there. We can find this in the midrash Tanchuma:
And Abarbanel ties this midrash from Rabbi Avahu citing Rabbi Shmuel bar Nachmani into his beautiful explanation of just how the acceptance of the covenant can work for the next generation. If ain chavin leadam shelo befanav, and ain adam morish shevua levanav, how can they accept this on behalf of subsequent generations? He answers that if someone accepts a loan, of course he and all generations afterwards are obligated to pay the debt, and he accepts upon himself and future generations this obligation, even though those future generations are not present. Similarly, an Eved Kenaani is an inheritance to the children, for he is a monetary purchase like the rest of the owner's assets.
אמר רבי אבהו בשם רבי שמואל בר נחמני:
למה כתיב, כי את אשר ישנו פה וגו' ואת אשר איננו פה? ש
לפי שהנשמות היו שם ועדיין גוף לא נברא, לכך לא כתיב בהן עמידה
When Hashem redeemed Israel from Egypt, they and all their possessions were from Him. And thus he acquired their very bodies, like Avadim Kenaaniim. And he also acquired their souls, for He gave them completeness of spirit at Matan Torah.
And various covenants were made as to the body and spirit (Abarbanel elaborates), and naaseh was with our bodies while nishma was with our souls. And now that He wanted to them another kindness, namely inheritance of the land, this brought about a requirement to enter with them into a new covenant; for the first was on the subjugation of their body and spirit, while the second was on inheriting the land. And the matter of this covenant was that they would not take possession of the land with their sword, nor would they inherit it from their fathers, but that Hashem would give it to them, not as a gift, but as a loan,
as is written that the land was not sold for perpetuity, for the land is Mine. And that they would be required to serve, in it, the Master of the land, and not serve another god except Him, for this would be a great rebellion and treason against Him.
Abarbanel continues for a while in his explanation, and concludes that behold, from the aspect of body, spirit, and the land of their sourjournings in which they sojourned, the fathers were obligated, and they are encompassed in the covenant which their fathers accepted. Not by virtue of the oath that they made, but rather by virtue of the servitude which they accepted when He took them out of Egypt, and by virtue of the Torah that they accepted as a pledge, and the chosen land which they accepted as a loan.
And, he concludes, upon this, Chazal certainly intended in the midrash Tanchuma in this parsha, when they said that the soulds were all present at the time of the covenant.
This is a beautiful pnimiyus interpretation of the midrash, or rather, a beautiful explanation and nice tie-in of the midrash into the explanation. Even so, I am neither convinced that this is the way that the eternal covenant was established -- who says that on a peshat level and realistically, covenants between Hashem and his people must work in accordance with halacha?! -- nor convinced that this was the true meaning of the midrash. After all, only one component is present, that of soul, but not of body and land; and Abarbanel fills in a lot of detail outside of the midrash, such that it is likely that many different interpretations of neshama could be read into this midrash. Even so, it provides a wonderful example of how extremely deep meanings can be encoded in seemingly trivial aggada.
I would analyze the midrash Rabba in a different manner. What in the text prompts this midrash? The pasuk, again, is:
There are two ways of parsing pasuk 14, and specifically
If the midrash is being driven by this textual reinterpretation, the conclusion should be that some people are here even though they are not here, which are the spirits of generations in the future. Taking it to be a cryptic midrash which should be reinterpreted in accordance with Abarbanel's chiddush undermines the very idea being deduced, that there are indeed people here who are not here. I admit there is still a way to allow both to work, but somewhat forced; and the straightforward interpretation works quite well with no difficulty. Unless of course one's theology makes future spirits unlikely or untenable; or that it would be a silly and superficial idea. If this is so, an alternative to reinterpretation would be to say that Rabbi Abahu and Rabbi Shmuel bar Nachmani maintained a different theology than I do.