What is the intent of some imams, such as Ahmad, when they say that a weak hadith is to be acted upon if there is no other hadith in the chapter?

The questioner : Shaykh, we hear that some imams, when no authentic hadith is found in a chapter but a weak hadith is found, say that it is to be taken in that chapter, like Imam Ahmad, for example. What is their intent by a weak hadith in this context?

The Shaykh : The intent of Imam Ahmad, as Ibn Taymiyyah explained in some of his books, is the good hadith known today. This is because the usage of the term "good hadith" was not widespread in the time of Imam Ahmad. At that time, hadiths were divided into two categories: authentic and weak. So, when it is reported from Imam Ahmad, occasionally, that a weak hadith is to be taken in a chapter if there is no other, he means the hadith that is established at the lowest level of establishment, which is the good hadith. These are technical terms known by those who engage in this science that has such terminology. When they say "good hadith" and "authentic hadith," they mean by these two terms that the establishment of the good hadith is less than the establishment of the authentic hadith. And what lies under this detail?
They mentioned in the science of hadith terminology that an authentic hadith is: what is narrated by a just and precise narrator from a like him from a like him to the end of the chain, and it is not anomalous and it is not defective. As for the good hadith, they defined it with the same definition in all its conditions, except that they said in the first or second condition—which is precision—that it is narrated by a just narrator whose precision is less than the precision of the trustworthy precise narrator. This refers to a narrator in the chain whose memory has some weakness, so his memory is not comparable to the memory of the trustworthy one whose hadith is authentic. That is to say: if there are two narrators, one of whom memorizes the narration or what he hears with good precision, so rarely, rarely does he make a mistake in his narration, whereas the narrator of the good hadith is a memorizer but he makes a mistake, occasionally, in his narration. They classify this type of narrators’ hadith as good because they observed some poor memory in their narrations, unlike the first one. If we know this detail, then in the hadith of the second narrator, whom they classified as good, there is some weakness relative to the first trustworthy narrator, in whose hadith there is no weakness. For this reason, they previously used to say that the second narrator’s hadith is weak, meaning relative to the first one.
And there is no doubt that when they used the terms "authentic" and "weak," it was settled in their minds that authenticity has degrees. The lowest degree of authenticity was later termed the good hadith. This terminology emerged shortly after the time of Imam Ahmad. Imam al-Bukhari, who was a student of Imam Ahmad, used it occasionally, and the student of Imam al-Bukhari, al-Hafiz al-Tirmidhi, popularized its usage. So, Ahmad’s statement of the permissibility of taking a weak hadith if there is no other in the chapter was understood to mean that the intended "weak hadith" is one that is weak relatively compared to the authentic hadith... except the weak hadith in the later technical sense, which comes in the third rank: authentic, good, weak. This is the one whose poor memory is severe. The hadith of this narrator is unanimously agreed upon as not to be taken in rulings, as mentioned by Shaykh al-Islam Ibn Taymiyyah (may Allah have mercy on him) in some of his fatwas.