Celebrating the first night of Pesach without a recounting of the story of the Exodus seems unfathomable today, but it is possible that until the time of the Rambam there was no consensus that there is any Biblical obligation to do so. Many Geonim do not count telling the story as a distinct commandment in their Books of Mitzvot, apparently assuming that remembering the event sufficed. Others do assume that a verbal retelling is necessary, but only if a child who is present questions. It is first the Rambam who asserts that even without children, it is incumbent upon every individual to verbally retell the story. In the post-Maimonidean era, the Rosh raises a third approach to the commandment, suggesting that it is action oriented. The story is retold through eating the Pesach sacrifice and thereby re-enacting the original Pesach. These various understandings of the obligation reflect distinct approaches to learning and service of God in general: intellectual, verbal and experiential. It might be that a combination of all three modes is the ideal way to fulfill the obligation..
No Biblical Obligation
There is no Biblical obligation to tell the story of the Exodus on Pesach night. The verses which seem to obligate one are understood to refer to either cognitive actions, other days of the year, or parts of other obligations.
Shemot 13:3 – "זָכוֹר אֶת הַיּוֹם הַזֶּה" – This approach might dismiss this verse as a source for the obligation on several grounds:
זכירה בלב – The verse uses the verb "זָכוֹר" rather than "אמור" (or the like) and thus might refer to remembering in the heart and not recounting via speech.
Heading – It is also possible that the phrase should be understood as a heading for what follows rather than an independent obligation. The nation is told to "commemorate the day you left Egypt" and the rest of the unit then explains how to do so: don't eat leavened bread, do the Pesach service, and celebrate Chag haMatzot for seven days.
Obligation for the rest of the year – In addition, it is not clear if the verse refers to remembering specifically on Pesach or throughout the year. See Mekhilta DeRabbi Yishmael13:313:14About Mekhilta DeRabbi Yishmael Shemot who reads this phrase as being the source for remembering the Exodus daily.3
Devarim 16:3 – "לְמַעַן תִּזְכֹּר אֶת יוֹם צֵאתְךָ מֵאֶרֶץ מִצְרַיִם כֹּל יְמֵי חַיֶּיךָ" – These sources would likely reject this verse as a source since the word "לְמַעַן" suggests that the remembering mentioned in the verse is not a commandment in and of itself, but simply the purpose of the obligation to eat matzah mentioned earlier. In addition, the verse speaks of remembering rather than telling, and specifies that this commemoration should take place "כֹּל יְמֵי חַיֶּיךָ" rather than on Pesach night specifically.4
Shemot 12:27 / 13:8 – "וַאֲמַרְתֶּם זֶבַח פֶּסַח הוּא"/ "וְהִגַּדְתָּ לְבִנְךָ... לֵאמֹר" – These verses explicitly mention speech5 and clearly refer to Pesach night. However, as they both speak of the recounting as accompanying the eating of the Pesach sacrifice, these sources might suggest that the telling is subsumed under the larger mitzvah of eating, and therefore does not constitute its own obligation. Alternatively, the recounting is simply the purpose of the eating and therefore is a קיום המצוה but not an independent obligation. It should be noted as well that both verses only speak about telling the story to children who ask, and say nothing of a case in which no one questions.
Shemot 13:13 – "וְאָמַרְתָּ אֵלָיו בְּחֹזֶק יָד הוֹצִיאָנוּ" – Though this verse directs one to verbally speak about leaving Egypt, its context is not Pesach, but the redemption of the first born. Thus the verse says nothing about any obligation on Pesach night.
Devarim 6:21 – "וְאָמַרְתָּ לְבִנְךָ עֲבָדִים הָיִינוּ" – The context of this command is also unrelated to Pesach. It speaks, instead, of a general obligation to explain to one's children that the reason to keep Hashem's commandments is the fact that He freed the Israelites from slavery in Egypt.
"כל המספר" versus "כל המרבה לספר" – In Seder R. Amram Gaon, the text of the haggadah reads, "וְכָל המספר בִּיצִיאַת מִצְרַיִם הֲרֵי זֶה מְשֻׁבָּח ", instead of "וְכָל הַמַרְבֶּה לְסַפֵּר". This might support the idea that many Geonim did not count the telling of the story as a Biblical commandment. The very act of telling the story (not the elaboration upon it) is praised because it was not obvious that everyone would do so.
No blessing on haggadah – This position would explain there is no blessing over the telling of the story in the haggadah because there is no Biblical obligation to do so.
Focus of the evening – According to this position, the purpose of all that is done on Pesach might be to elicit an internal response. One need not say anything out loud, but one should think of the miracles of the Exodus and feel them in one's heart.
Telling Through Eating
The obligation to tell the story of the Exodus does not necessitate a verbal retelling, but is rather fulfilled through the act of eating the Pesach sacrifice. By eating the lamb with matzah and maror, one re-enacts the original Pesach thereby effectively recounting the essence of the story.
Source for the mitzvah – According to this approach, the verses in Shemot 12:26-27, Shemot 13:3-8 and Devarim 16:1-3 which speak of remembering the Exodus as one offers the Pesach teach that the mitzvah is action-oriented and the recounting takes place through the act of eating itself. The language of "וַאֲמַרְתֶּם זֶבַח פֶּסַח הוּא לַי"י" or "וְהִגַּדְתָּ.... לֵאמֹר" is not understood literally to refer to verbal expression, but to the conveying of information.
Comparison to other mitzvot – The Rosh compares the obligation to remember the Exodus when eating the Pesach to other mitzvot which similarly commemorate the enslavement and redemption, such as redemption of firstborns or celebration of Shabbat and holidays. In each case one need not actively speak about the Exodus; the fulfillment of the mitzvah itself serves to commemorate it.
No blessing on haggadah – The Rosh explains that there is no blessing made over telling the story in the haggadah, just as there is no blessing over commemorating the Exodus when redeeming a firstborn. In both cases, the blessing is instead made over the active part of the mitzvah (eating of matzah and maror, redeeming of the son, etc.) which itself constitutes fulfillment of the commemoration.
Era of the Mikdash – The Rosh's understanding of the commandment is very logical for the time of the Mikdash, when the Pesach was the highlight of the evening. Each family, in sacrificing a lamb and eating it with matza and maror, effectively re-enacted the original Pesach offering. As such there was no real necessity to verbally relay the story. That said, it was also very likely that the act itself would have elicited questions and led into a discussion of the Exodus.
Different modes of learning – According to the Rosh, the night is experiential in nature. If "a picture is worth a thousand words," then a dramatic re-enactment might be worth even more.
Verbal Recounting
There is a Biblical obligation to verbally tell the story. This position subdivides regarding whether the obligation exists only if a child questions or even if not:
Only If Child Asks
The obligation only entails responding to a child who asks. Had there been no child, a parent would not have to recount the story.
Source for the obligation – As the only verses which clearly speak of verbally retelling the story of the Exodus specifically on the eve of the 15th (Shemot 12:27 and 13:8 ), do so only in the context of responding to a child, this position limits the obligation to such a scenario.
Purpose of commandment – The purpose of the commandment would seem to be to ensure the continuous transfer of tradition. As only the first generation witnessed the miracles first-hand, it is vital that they pass on what they saw, in an unbroken chain, so that later generations will continue to believe.6
Even Without Children
Every individual must tell the story of the enslavement and redemption, regardless of whether or not a child is present.
Source for the obligation – Rambam looks to two verses, Shemot 13:3 ("זָכוֹר אֶת הַיּוֹם") and 13:8 ("וְהִגַּדְתָּ לְבִנְךָ"), which when combined, teach that there is an obligation to verbally retell the story, even when no child asks. The two verses complement each other, each compensating for what the other lacks. While Shemot 13:8 refers to a verbal retelling which takes place on the eve of the 15th, it limits the obligation to responding to a child. Shemot 13:3, on the other hand speaks only of remembering and is unclear as to the timing of the commandment, but expands the obligation to all.
"זָכוֹר אֶת הַיּוֹם הַזֶּה" – Rambam maintains that the word "זָכוֹר" refers to speech rather than to memory by comparing it to the command " זָכוֹר אֶת יוֹם הַשַּׁבָּת", which is also understood to refer to a verbal action, the recitation of kiddush.
Different modes of learning – According to the Rambam, the focus of the evening is not the experience of eating, but speaking and learning. This matches Rambam's general approach to mitzvot where he favors use of the intellect in one's service of Hashem and fulfillment of obligations.
Influence of the Destruction of the Mikdash – It is possible that after the destruction of the Mikdash, when there was no longer a Pesach offering, there was a need to change the focus of the evening from the previous emphasis on the sacrifice and meal to the story itself. This is comparable to the general shift from sacrifice to prayer.
"כל המספר" versus "כל המאריך לספר" – Since the Rambam maintains that there is an obligation to tell the story, it would be odd to praise someone for simply fulfilling a Biblical obligation. Thus, it is logical that he prefers the version of the haggadah that praises one for expounding at length about the story (וְכָל הַמַּאֲרִיךְ לְסַפֵּר בִּיצִיאַת מִצְרַיִם הֲרֵי זֶה מְשֻׁבָּח).