In debating, sometimes one side makes an argument against the other side based on the premises that the other side accepts. The purpose of such argument is to show that the other side is wrong even based on what he/she maintains. Usually this kind of argument has a condition which is not in reality true, such as “if I lied,” “if I went astray,” etc. Imam Ali (AS) in his letter to Muawiya presents an argument based on what Muawiya believes to show that Muawiya is not able to question Amir al-Mu'minin's (AS) rule, even on the basis of his own logic.
Moreover, what the Imam says in his letter about God being pleased with the rule of the first three caliphs has a condition, i.e., the real consensus of the Emigrants (Muhajerin) and the Helpers (Ansar), which never took place.
Before giving the answer, let us call your attention to a story of prophet Ibrahim (AS). The Qur'an tells us that after Ibrahim (AS) secretly destroyed the idols, the people, knowing him for criticizing idolatry, quickly blamed the destruction of the idols on him. They asked him “Did you do this to our gods?” Ibrahim (AS) replied: “The big idol did it. Go ask your gods, if they can speak.” The idolaters were surprised by this answer and did not have anything to say.
The method that Ibrahim (AS) used here was using the idolaters' own beliefs -- which were false, of course -- to organize an argument against them. The idolaters considered themselves and the whole world to be created by the idols; the objects that were destroyed by Ibrahim (AS) and could not even defend themselves. Now, Ibrahim (AS) tells them to refer to the big idol which was not destroyed to ask him about what had happened.
This does not mean that Ibrahim (AS) really believed that the big idol was responsible for destroying the other idols or that it had the ability to speak, but he was only using the other side's beliefs to argue against them. The sixth Imam (AS) says in this regard, “although Ibrahim's saying was apparently a lie, it was not really a lie, because he had conditioned his saying to the speaking of the idols, which was not possible.” Similarly, Amir al-Mu'minin (AS) in his letter to Mu'awiyah uses the latter's false beliefs to argue against him. Without lying, Imam Ali (AS) shows that Mu'awiyah's claims are, even based on his own logic, false.
He states in this letter,
"Verily those who swore allegiance to Abu Bakr, Umar and Uthman have sworn allegiance to me on the same basis on which they swore allegiance to them. (On this basis) he who was present has no choice (to reconsider), and he who was absent has no right to reject. And consultation is confined to the muhajirun (the Emigrants) and the ansar (the Helpers). If they agree on an individual and take him to be Caliph, it will be deemed to mean Allah's pleasure.
Now, let us reflect on the stages of the Imam's argument:
1. He states that those who paid allegiance to him are the same people, based on whose allegiance the former caliphs justified their rule. Therefore, if Mu'awiyah accepts their rule on this account, he cannot reject Imam Ali's rule.
2. Mu'awiyah maintains that -- albeit only to justify the rule of the past caliphs who took on power without the consent or awareness of the whole Muslim community -- only the Emigrants and the Helpers must be consulted. Because this idea, although lacking evidence, is accepted by Mu'awiyah and his party, the Imam uses it to argue that even based on this, Mu'awiyah cannot object that he did not participate in the election of the Imam as the caliph, since he was not from the Emigrants or the Helpers, but from the Tulaqa', the people who, while deserving enslavement, were granted freedom by the Prophet (PBUH) after the conquest of Mecca.
3. In another part of the Imam's argument, to which your question mainly refers, he states that if all the Emigrants and the Helpers elected a person as the imam, God would be pleased with the imamate of that person.
Now we need to focus on this statement which appears at first sight to be in conflict with the Shiite standpoint:
3.1. The Shiite logic:
As was explained before, in the Shiite perspective, in order for a legitimate state to be established, the support of the people is necessary. However, not every state supported by the people is believed to be legitimate; rather, legitimacy is determined by other criteria, such as being based on God’s will and on truth.
Thus, according to the Shiite belief, Imam Ali (AS) was always the rightful caliph– regardless of whether the people accepted or rejected him -- because he was appointed by God and his messenger (PBUH) to be the Prophet’s (PBUH) successor
3.2. The Sunni logic:
In the Sunni perspective, the consensus of Muslims is the base for legitimacy and is believed to entail God’s pleasure. However, the Sunnis define consensus as the agreement of a small group of Muslims. This definition is not accepted by the Shiites, because it is not a real consensus, comprising the consent of all people. Yet, if the Shiites would accept a real consensus of the Emigrants and the Helpers, their consensus would have certainly entailed God’s pleasure, because Imam Ali (AS) – no doubt one of the greatest Emigrants -- would have been among them. And, for the Shiites, his consent would in all cases entail God’s consent – be it in agreement with other people’s consent or not. This delicate point can easily explain the Imam’s statement that if the Emigrants and the Helpers unanimously accept somebody as the imam, his imamate would entail God’s pleasure.
Historically speaking, however, such consensus and such unanimous agreement never took place. In the process of the election of the first caliph, at least in the preliminary stages, Ali (AS), Zubayr (two prominent figures of the Emigrants) and Sa‘d b. ‘Ubāda (a prominent figure of the Helpers) did not pay allegiance; thus, no real consensus took place. The election of the second caliph was not really an election, but an appointment by the former caliph. According to history, even the committee of the six people who were appointed to elect the third caliph did not reach a consensus, with Imam Ali (AS) on one side and ‘Uthmān on the other . Thus, the third caliph’s rule was not based on any consensus either. Therefore, no consensus had occurred to entail or not entail God’s pleasure.
Thus, what Imam Ali (AS) meant by the consensus was an all-encompassing consensus of the Emigrants and Helpers. However, Mu‘awiyah could not reject the Imam’s caliphate based on not paying allegiance on the part of people like Sa‘d b. Abi Waqqas, Abdullah b. Umar, and others, because according to his own logic, the imperfect consensus was also enough. And obviously Amir al-Mu’minin (AS) had the support of the majority of the Muslims.
Therefore, Imam Ali’s argument was a sound argument both from the Shiite perspective and from Mu‘awiyah’s standpoint.
In other words, the Imam’s statement is an equivocal statement; one interpretation of it is not in contradiction with the Shiite beliefs, because no consensus took place in the cases of the former caliphs to entail God’s pleasure or not; the other interpretation, accepted by Imam Ali’s (AS) opponents, is in fact to their detriment.
This issue will be clearer if we remember that Mu‘awiyah claimed legitimacy on the basis of being appointed by former caliphs whose proof for being rulers was by no means stronger than Imam Ali’s (AS) proof.
In conclusion, it should be noted that this kind of argument is used in the Qur’an several times. We mention two examples here:
1. Do they say,” He has fabricated it?” Say,” Should I have fabricated it, then my guilt will be upon me, and I am absolved of your guilty conduct.” (11:35)
2. Say,” If I go astray, my going astray is only to my own harm, and if I am rightly guided that is because of what my Lord has revealed to me. (34:50).
Obviously, the former verse does not indicate a confession by the Prophet (PBUH) to lying -- and we seek refuge to Allah from such though; nor does the latter denote that he had gone astray. Rather, it is the debating method that sometimes requires conceding to some of the other side’s beliefs in order to show that the other side cannot establish their claim even based on their own beliefs.