Must a layperson keep to a single school of law?

In the "benefits concerning the fatwa" of I'lam al-Muwaqqi'in, Ibn Qayyim al-Jawziyya asks whether a layperson must adhere to one of the known schools. He records two positions and holds that the layperson need not. ¹

His argument is that nothing is obligatory except what God and His Messenger made obligatory. Neither of them obliged anyone to adhere to the school of one man of the community and follow him in the whole of religion, and the best generations had no such affiliation. ¹ In his view a layperson cannot really have a school at all. A school belongs to someone with some capacity for reasoning and inference, or someone who has read a book on its rulings and knows its imam's fatwas. Someone who says "I am a Shafi'i" or "a Hanbali" without this does not become one by saying so, any more than he becomes a jurist or a grammarian by claiming it. ^{1 2}

He says that no imam of Islam required anyone to take all of one imam's views and leave everyone else's. He also expresses wonder that the schools of the Companions, the Successors and all the other imams should have died out, leaving only four. ² He states that once a hadith is sound it must be acted on, whatever the region of the one who holds it. He compares this with the readings of the Qur'an: a Muslim is not bound to the seven readings when another reading matches the script of the Mushaf and Arabic usage and has a sound chain. ³ He adds a limit, however: the layperson may not hunt for the concessions of the schools and take what suits him from whichever school has it, but must follow the truth as far as he can. ³

Ibn al-Qayyim mentions those who held that a layperson must adhere to the school of some scholar, and, further still from the truth in his view, those who held that he must adhere to one of the four schools. ² He reports their argument: whoever believes his school to be right must act on his belief. He answers that this would make it forbidden to ask a mufti of another school, and would oblige a person to leave a text when it sides with someone other than his imam. ²

The holders of the obligation view and their own arguments have not yet been added by name and in their own words.