Since these holy personalities are guides of humanity...
Since these holy personalities are guides of humanity, they themselves acted in accordance to laws of Shariat and practiced the best of morals in such a way that perfections of humanity were personified in them. They also called the people to same beliefs and morals. Hence all beliefs, acts, morals and traits of character can be judged on their criterion; thus they are considered to be the balance of deeds. The same interpretation is mentioned in some traditional reports.
Imam Ja’far Sadiq (as) said in the interpretation of the verse: وَنَضَعُ الْمَوَزِينَ الْقِسْطَ لِيَوْمِ الْقِيَمَةِ And We will set up a just balance on the day of resurrection… (21:47) “They are the prophets and their successors.”[^7] Imam Sajjad (as) said: “Ali (as) is the chief of faith and the balance of deeds.”[^8] Imam Ja’far Sadiq (as) said: “By Allah, Ali is the straight path and the balance of deeds.”[^9] The Messenger of Allah (S) said: “I am the balance of knowledge and Ali is the two pans of balance, Hasan and Husain are its cords and Fatima is the connecting rod between them and the Imams after Hasan and Husain would be the criterion of recognition of their friends and their enemies.”[^10] Therefore the holy beings of prophets and their infallible successors can also be interpreted as the balance to weigh the deeds of the people of the community, but think upon it for a moment and you will see that that the second reason also returns to the first cause, since the beings of the prophets and successors are personification of beliefs, morals and good character of religion, they are introduced as balance; thus the balance is in fact a criterion to weigh beliefs and deeds and laws of religion.
And the deeds of every person can be weighed in it and he could be rewarded and punished accordingly. In this weighing, beliefs and deeds of a person would be placed in one pan and the other pan contains all beliefs, morals and laws of religion. From some verses of Quran it can be concluded that man has not one, but many balances.
وَ الْوَزْنُ يَوْمَئِذٍ الْحَقُّ فَمَن ثَقُلَتْ مَوَ زِينُهُ فَأُوْلَئِكَ هُمُ الْمُفْلِحُونَ \* وَمَنْ خَفَّتْ مَوَ زِينُهُ فَأُوْلَئِكَ الَّذِينَ خَسِرُواْ أَنفُسَهُم بِمَا كَانُواْ بَِايَتِنَا يَظْلِمُونَ And the measuring out on that day will be just; then as for him whose measure (of good deeds) is heavy, those are they who shall be successful.