The field of al-Jarḥ wal-Taʿdīl (disparagement and appraisal) of men is a lofty science and is founded upon a methodology (and principles) that keep it within the bounds of truth and justice, with speech that represents the realities, and where error, oversight, exaggeration, harshness and laxity—which are inevitable as no one is infallible and free of error and everyone is subject to human limitations—are removed or minimised. This ensures that the conveyance of truth is not compromised through erroneous judgements regarding men.
Shaykh Rabīʿ bin Hādī (رحمه الله) said:
You must (follow) the knowledge of the Salaf and their methodology, their methodology in disparagement and appraisal (al-jarḥ wal-taʿdīl). We have found Yaḥyā bin Maʿīn—about whom it is said that he was harshest in disparaging (men)—we found leniency with him (in relation to some). And we found scholars who oppose him, from them are those who are greater than him (in status) and from them are those who are lower. How many did he disparage and they opposed him (therein), and how many did he appraise and they opposed him? And Aḥmad bin Ḥanbal appraised and disparaged, yet they opposed him in appraisal and disparagement. Why? Because they have a methodology (that they adhere to), it is the methodology (that is the criterion and is returned to), not so-and-so. It is obligatory for so-and-so to judge by the principles of this methodology.
Ahl al-Sunnah wal-Jamāʿah are distinguished in that in all fields of knowledge, they take the stataments of men in accordance to the methodology and the principles of the discipline they speak in, and not on account of the status of the scholar. This includes the field of disparagement and appraisal (al-Jarḥ wal-Taʿdīl). The acceptance of the disparagement or appraisal returns to the methodological principles of this discipline. Thus, a lax appraisal that is contrary to the facts is rejected, just as a harsh disparagement that is contrary to the facts is rejected. Likewise, a disparagement of a trustworthy person is not accepted unless it is evidenced and detailed and so on.
This is how the scholars in this field behaved, and this is why not every disparagement of a scholar was accepted by others, and not every appraisal of a scholar was accepted by others, even if he may have been correct in most of what he said.
This is indicative of the fact that the affair returns back to evidence and to the methodology and the principles of the discipline and not because of the status of a scholar, as Shaykh Rabīʿ (رحمه الله) explained. Had this not been the case, then everyone would argue, “My scholar is greater than yours” and “So-and-so is much more knowledgeable than so-and-so”, and the affair would return to the status of scholars, not to evidences and agreement with factual realities, and the discipline would be compromised.