In Nahj al-Balaghah this matter has been rationalized on three principles:
1) The will and designation of the Holy Prophet.
2) The worthiness of Amir al-¬Mu’minin, (A.S.) and the fact that the robe of Caliphate was suitable only for him.
3) The close genealogical and spiritual relation that Hazrat had with the Holy Prophet (S.A.W.A.).

1) Will And Designation
Some reckon that the matter of designation has not been mentioned in Nahj al-Balaghah at all and what is referred to is only the matter of competency and worthiness. Such reckoning is incorrect because first of all, just as it was mentioned in the previous chapter, in Sermon No.2, ‘Ali (A.S.) very explicitly says about Ahl al-Bayt: وَ فِيهم الوَصِيَّةُ وَ الْوِراثَةُ i.e. the testament and the inheritance of the Holy Prophet (S.A.W.A) are concerning them.
Secondly, ‘Ali (A.S.) on many occasions has spoken about his right in such manner that one cannot explain them save by the mailer of appointment and clarification of his right to the seat of Caliphate by means of the Holy Prophet (S.A.W.A.). In those talks, ‘Ali (A.S.) does not wish to say that why they have kept him aside and appointed others in spite of comprehensiveness of the conditions. By his talks it is meant that they have robbed his certain and definite right. It is evident that it is only by designation and previous appointment by the Holy Prophet that one can speak of certain and definite right. Competency and worthiness establishes potential right and not actual right and in the case of potential right it is wrong to speak of snatching away of certain and decisive right.
Now we shall mention some instances where ‘Ali (A.S.) reckons the Caliphate to be his just right. For example it is mentioned in sermon No.6 that at the beginning of his Caliphate when he became aware of the revolt of Ayesha, Talha and Zubair and decided to confront them, he said (after mentioning a few points on the current state of affairs):

فَوَاللهِ مَا زِلتُ مَدْفُوعاً عَنْ حَقِّي، مُسْتَأْثَراً عَلَيَّ، مُنْذُ قَبَضَ اللهُ تعالى نَبِيَّهُ صلى الله عليه وآله حَتَّى يَوْمِ النَّاسِ هذَا

“By Allah, from the day the Prophet passed away until today, I have been continually deprived of my right, with others being given preference to me.”
In Sermon No.170 which is in effect not a sermon and it would have been better if Sayyid Razi (may Allah exalt his position) brought it under the section of maxims of ‘Ali (A.S.), he narrates an incident which is as follows: ‘¬In the presence of some people, a person approached me and said: “O son of Abu ¬Talib! You are greedy of Caliphate.”
I replied:

بَلْ أَنْتُمْ وَ اللَّهِ لَأَحْرَصُ وَ أَبْعَدُ وَ أَنَا أَخَصُّ وَ أَقْرَبُ وَ إِنَّمَا طَلَبْتُ حَقّاً لِي وَ أَنْتُمْ تَحُولُونَ بَيْنِي وَ بَيْنَهُ وَ تَضْرِبُونَ وَجْهِي دُونَهُ فَلَمَّا قَرَّعْتُهُ بِالْحُجَّةِ فِي الْمَلَإِ الْحَاضِرِينَ هَبَّ كَأَنَّهُ بُهِتَ لَا يَدْرِي مَا يُجِيبُنِي بِهِ

“Rather, you are, by Allah, greedier although more remote, while I am more suited as well as nearer. I have only demanded it as my right while you are preventing me from it and turning me away from it.” When I rebuked him with (my) argument (and proof), he rose up as if bewildered, not knowing what reply to give me about it.’
It is not known who the person making such an objection was, and when this objection was made. Ibn Abi al-Hadeed says:- The person who objected was Sa’d b. Abi Waqqas and it was on the day of the “Shura” (Assembly). Thereafter he says: However the Imamiah believe that the person who objected was Abu ‘Ubaydah and it was on the day of Saqifah.
Following these sentences, it has been mentioned that ‘Ali (A.S) said:

اللهم اني استعديك علي قريش و من اعانهم فانهم قطعوا رحمي و صغَّروا عظيم منزلتي و اجمعوا على منازعتي امرا هولي

“O Allah, I seek Thy succor against the Quraysh and those who are assisting them because they have denied me (the rights of) kinship, have belittled my high position and are united in opposing me on a matter (Caliphate) which is my right.”
Below these sentences, lbn Abi al-Hadeed says:- “Sentences like the above sentences of ‘Ali based upon grievance against others and the fact that he has been unjustly deprived of his certain right has been narrated to the extent of successive transmission and is in conformity to the views of Imamiah who say that ‘Ali has been appointed by indisputable designation and nobody had any right to take the seat of Caliphate under any circumstances.
Ibn Abi al-Hadeed himself is the supporter of ‘Ali’s superiority and excellence. According to him, inasmuch as the sentences of Nahj al-¬Balaghah impart the purport of the Imam’s priority, it is needless to explain it. However the above sentence according to him needs to be justified since it has been stipulated that the Caliphate has been the special right of ‘Ali (A.S.) and this cannot be conceived but by appointment and the fact that the Holy Prophet (S.A.W.A.) has fixed the responsibility and clarified the right on behalf of God.
One of the companions of ‘Ali (A.S.) belonging to the tribe of Bani-Asad asked ‘Ali (A.S.) as such:

كيف دفعكم قومكم عن هذا المقام و انتم احق به

“How is it that your people have deprived you of this station (Caliphate) whereas you were most worthy of it?”
Amir al-Mu’minin (A.S.) responded to this question and his reply is the same as what has come down in Sermon No. 160 in Nahj al-Balaghah. ‘Ali (A.S.) explicitly states that greed and avarice on the one hand and negligence (of the interests of the people) on the other hand were the only factors leading to such a situation.

فانها كانت اثرة شحت عليها نفوس و سخت عنها نفوس آخرين

“It (the matter of the Caliphate) was an act of appropriation (of something) for which some became greedy and others relinquished”.
This question and answer took place during ‘Ali’s Caliphate at the time when he was involved (in opposition) with Mu’awiya and his wiles. Amir al-Mu’minin (A.S.) disliked discussion of this matter under such circumstances. So, before replying, he reproved him saying that every question has its own appropriate place and time and that it was not proper to discuss the past at that moment. He told him that the issue of the day was that of Mu’awiya.

و هلم الخطب في ابن ابي سفيان...

“Come now to the affair of Ibn Abi Sufyan (Mu’awiya)…”.
Anyhow, just as it was his usual temperate method, he did not refrain from answering and clarifying the past realities. In the Sermon of Shiqshiqiya, he explicitly says:

أرى تراثي نَهباً

“I witnessed the plundering of my inheritance..”.
It is obvious that by inheritance he did not mean family or kinship inheritance but spiritual and divine inheritance.

2) Virtue and Superiority
The second issue after the subject of distinct designation and definite right is the subject of virtue and superiority. This matter too has repeatedly been mentioned in Nahj al-Balaghah. In the Sermon of Shiqshiqiya, ‘Ali (A.S.) says:

و اما و الله لقد تقمصها ابن ابي قحافة و انه ليعلم ان محلي منها القطب من الرحي ينحدر عني السيل و لا يرقي الي الطير

“By Allah, the son of Abu Quhafah (Abu Bakr) dressed himself with it (the Caliphate) while he certainly knew that my position in relation to it was the same as the position of the axle in relation to the mill. The torrent (of knowledge and virtue) flows down from me and birds cannot rise up to (the loftiness of) my position.”
In Sermon No.195, he first recalls the level of his submission and faith towards the Holy Prophet (S.A.W.A.) and then recounts his sacrifices and help in various instances. Thereafter, he narrates the incident of the demise of the Holy Prophet when his head rested on his chest, and then mentions the event of giving ‘Ghusl’ (Ablution) to the Holy Prophet (S.A.W.A.) with his own hands while the angels were assisting him in this work and he could listen to their humming voices and perceive the manner in which a group amongst them would come and another group amongst them would depart while sending salutations upon the Holy Prophet (S.A.W.A.). And right until the last moment of the Holy Prophet’s burial, the murmuring of the angels did not cease reaching ‘Ali’s ears.
After recalling his various special positions - the position of his submission and belief, (contrary to some of the other companions) his unequalled sacrifices, his relationship with the Holy Prophet (S.A.W.A.) and the fact that the Holy Prophet breathed his last on his very lap, he says:

فمن ذا أحق به مني حيًا و ميتًا

“Thus who has greater rights with him than I, during his life or after his death?”

3) Relationship and Lineage
Just as we are aware, moments after the Holy Prophet’s demise, Sa’d b. ‘Ubadah al-Ansari claimed the Caliphate and a group amongst his tribe gathered around him. Sa’d and his followers chose the place of Saqifah for this very purpose. However Abu Bakr, ‘Umar and Abu ‘Ubaydah al-Jarrah reached there and prevented the people from falling for Sa’d b. ‘Ubadah’s plot and instead took allegiance from the people in Abu Bakr’s favor.
In this assembly, words were exchanged between the Muhajirin and Ansar and various factors played their role in determining the ultimate fate of this gathering.
One of the so-called winning cards, which the Muhajirin and the supporters of Abu Bakr played, was that the Holy Prophet (S.A.W.A.) was from Quraysh and that they themselves belonged to the Holy Prophet’s clan. In the commentary of Sermon No.65, Ibn Abi al-Hadeed says: ‘Umar addressed the Ansars: “The Arabs will never agree to have you as the head of the state since the Prophet was not from amongst you. Certainly the Arabs will not oppose if the Caliphate is allowed to one in whose house rests the Prophethood .... Who can oppose us in connection with rulership and inheritance of Muhammad whereas we are his ones and his relatives.”
Again, just as we are aware, during that very moment, ‘Ali (A.S.) was fulfilling his personal responsibility of burying the Holy Prophet’s body. After the termination of this event, ‘Ali (A.S.) enquired from those present in that gathering about the reasoning put forth by both the opposite groups and after listening to them he criticized and rejected the reasoning of both the groups. In this connection, the sayings of ‘Ali (A.S.) are the same which Sayyid Razi has mentioned in Sermon No.65.
‘Ali (A.S.) asked: “What did the Ansars say?” They replied: “There should be one chief from us and one from amongst you.” Amir al-Mu’minin said: “Why did you not argue that the Prophet (S.A.W.A.) had instructed that whoever is good amongst the Ansars should be treated well and whoever is bad he should be forgiven!”
The people said: “What proof is there against them in this?”
Amir al-Mu’minin said: “If the government was (to be) from among them, there would have been no instruction (to others) for them.” That is, giving instructions to others about them is proof that the government belongs to other than them.
Then he said: “What did the Quraysh plead?” The people said: “They argued that they belong to the lineal tree of the Prophet.” Then Amir al-Mu’minin said:

احتجوا بالشجرة و اضاعوا الثمرة

“They defended themselves with the plea of the tree but neglected the fruit.”
In other words, if the tree indicates true relationship where others are the branches of that tree and the Prophet is one of those branches, then Ahl al-bayt of the Prophet are the fruits of those branches.
In Sermon No.160, a part of which was narrated earlier, there is a repertoire of questioning and answering between one Asadi man and ‘Ali (A.S.) wherein the latter argues in connection with the matter of relationship too. His expression is as such:

أما الِاسْتِبْدَادُ عَلَيْنَا بِهَذَا الْمَقَامِ وَ نَحْنُ الْأَعْلَوْنَ نَسَباً وَ الْأَشَدُّونَ بِالرَّسُولِ الله صلى الله عليه واله نَوْطاً

“As for the predominance over us in this station (of Caliphate), when we are the loftiest in lineage and the strongest in relationship with the Messenger of Allah (s)….”
The reasoning of relationship set forth by ‘Ali (A.S.) is a kind of logical dispute. Taking into account the fact that others had fixed the matter of genealogical relationship as the main argument and proof, ‘Ali (A.S.) would say: “Leaving aside all other matters like designation, virtue and worthiness, if we take into consideration the same lineage and relationship which others wish to rely on, I would still be more worthy than the other claimants to the seat of Caliphate.”