What are the conditions for independent ijtihad, and did the Four Imams reach this rank?

For a mujtahid (independent jurist) to qualify, he must be familiar with the sources of legal rulings from the Qur'an, Sunnah, Ijma' (consensus), Istishab (presumption of continuity), and Qiyas (analogy). He must also know what is preponderant when there is a conflict and prioritize what ought to be prioritized. Justice (adalah) is not a condition for the essence of ijtihad itself, but rather for the acceptance of his fatwa (legal opinion). It is not required to memorize the verses and hadiths related to rulings; rather, knowing their locations suffices. It is a condition that the mujtahid be aware of the abrogating and abrogated texts (nasikh and mansukh), the points of consensus and disagreement, and that he knows that what he uses as evidence is not abrogated and that no consensus has been reached on the issue in question. He must also have knowledge of the general and the specific (‘amm and khass), the absolute and the restricted (mutlaq and muqayyad), the explicit text and the apparent meaning (nass and zahir), the interpreted and the ambiguous (mu'awwal and mujmal), the clear and the explained (mubayyan), the stated and the inferred meaning (mantuq and mafhum), the unequivocal and the ambiguous (muhkam and mutashabih). Furthermore, he must know which hadiths are suitable for argumentation, be able to distinguish between the authentic, the good, and the weak (sahih, hasan, and da'if), and possess the necessary knowledge of grammar and language to understand the discourse. The four Imams are, by consensus, pillars of ijtihad.