Authority
This source of knowledge is about supporting a conclusion by citing other person who is expertise in the certain area. Authority is often used and it is not the primary source. However, the authors suggest that sometimes this source of knowledge leads to unreliable knowledge because authorities can be wrong as well. Therefore the authors suggest that about before citing on authority, it should be ensure that person is really an expert in that particular field of knowledge, and investigate the prior source of knowledge if there are disagreements between different authorities, therefore citing the authority on the one that can be assured that it can be investigate the source.