Who Are the People of the Madhhab?

Answer (and Allah is the granter of success):
The sum of the matter is this: the madhhab consists of the sayings of five
Imams:
 Imām al-Qāsim b. Ibrāhīm,
 his son Muḥammad b. al-Qāsim,
 his grandson al-Hādī Yaḥyā b. al-Ḥusayn,
 the two sons of al-Hādī: Aḥmad al-Nāṣir and Muḥammad al
Murtaḍā—peace be upon them all.
These are called the Imams of the Texts (Aʾimmat al-Nuṣūṣ).
Then came after them the Imams of Compilation and Derivation—Abū
al-ʿAbbās al-Ḥasanī, al-Muʾayyad bi-llāh, and Abū Ṭālib. They compiled
the madhhabs of the five Imams, systematized them, elaborated upon
them, branched out from them, and derived rulings—all in accordance
with what the words of the earlier Imams [the five Imams] entailed.
However, their derivations differed in some details and branches, according to their different understandings. This constitutes the second
stage of the madhhab.
Then came the third stage: rigorous scrutiny and examination of the
soundness of what the people of the second stage had established in
every branch and derivation, from the beginning of fiqh to its end.
Whatever they found to accord with the principles, foundations, and
texts of the earlier Imams, they marked with the well-known symbol 1 .
Whatever did not accord, they left unmarked.
Through the past centuries, scholars have continued to verify the
soundness of what their predecessors established. Whatever they found
in agreement with the texts of the five Imams, they confirmed and
marked as part of the madhhab. Whatever was not so, they attributed to
its author, saying: “al-Sayyid al-Muʾayyad bi-llāh said,” “Abū Ṭālib said,”
“Abū al-ʿAbbās said,” or “such-and-such jurist said.”
Objection: We find in the books of the madhhab that sometimes the
madhhab is reported in one issue, and then a contrary opinion is attributed
to al-Hādī. How then do you say that the madhhab is that of al-Hādī, his
grandfather, his two sons, and Muḥammad b. al-Qāsim?
Response: When the compilers and derivators studied the sayings of
those five Imams and examined them thoroughly, they extracted from
them principles and foundations. They then built upon these principles
and foundations and made them the basis for the derivations. Whenever
they found a statement from one of the five Imams that contradicted
these principles and foundations, they did not adopt it or establish it.
If you say: How could they attribute to the madhhab what they
established as the madhhab of al-Hādī, when al-Hādī himself disagreed
with it in some places?
We say: They deemed it permissible because they saw that al-Hādī,
after a thorough study of his words, relied upon principles and
foundations which he built upon—such as the principle that the default
state is purity. If they found him making a statement contrary to the
principles he relied upon, they adopted the principle and foundation that
he had built upon.
[The Preferable Course Is to Follow the Stronger Opinion] - As it appears to me, it is permissible for a follower to move between
the madhhabs of the scholars and Imams of the Ahl al-Bayt. Indeed, Zayd
b. ʿAlī (ʿa) said in his printed collected letters: “Our differences are a
mercy for you.” This means that the differences of the scholars of the Ahl al-Bayt in a matter are a breadth for the followers: no one of them is
bound to a single opinion, but has latitude and freedom to move between
them. - Even so, it is preferable for the follower to imitate the scholar he sees
as weightier. If the scholars appear equal to him, he takes the opinion
that seems stronger in his view. This is something a rational person is
guided to by the innate light of his reason.