Do hadith scholars require that the 'illah (operative cause of a ruling) be a defect?

The questioner : The 'illah (operative cause of a ruling) among hadith scholars: is it required that it be a defect, or do they use the term 'illah (operative cause of a ruling) in their definitions?

The Shaykh : ... Not every hadith that is described as having an 'illah (operative cause of a ruling) has a defecting 'illah (operative cause of a ruling). Similarly, not every confused hadith is considered a weak hadith. A confused hadith may have one of its aspects of confusion strengthened, in which case, based on this strengthening, the hadith either escapes being weak or its weakness is confirmed. Therefore, not every confusion in a hadith makes it defective, and not every 'illah (operative cause of a ruling) in a hadith makes it weak. This is mentioned in the science of hadith terminology.