What are the conditions for a jurisprudential disagreement to be considered valid?

Assessing a disagreement and judging whether it is permissible or not depends on examining its evidence. Thus, whatever is built upon valid proof, be it from established texts (thubūt), clear indications (dalālah), analogy (qiyās), or public interest (maslahah), is considered valid. Disagreements are of two types: forbidden and permissible. The forbidden type is that in which the proof is established by a clear text from the Book of Allah or the Sunnah of His Prophet, and disagreement in it is not permissible for one who knows it. The permissible type is that which allows for interpretation (ta'wīl) or analogy (qiyās). The decisive factor is the basis of the dissenting opinion; if its evidence is extremely weak, it is not considered, but if its basis is strong, then its disagreement is acknowledged. Distinguishing between a valid disagreement and an invalid one is the function of the mujtahidīn (those capable of independent legal reasoning). For those who are not specialists, a rough criterion is to observe what the majority of the Ummah (Muslim community) adheres to; if a new opinion contradicts it, it is generally not considered valid.