Sentence:  It is sometimes intimated that testimony which is direct and positive cannot legally be rebutted and overturned by presumptions less high than those which are juris et de jure in the absence of <WSD>impeachment</WSD> of the credibility of the witnesses. We are not prepared to accept that proposition unqualifiedly. 
Meaning of the word marked with <WSD> tags:  A demonstration in a court of law, or before another finder of fact, that a witness was ingenuine before, and is therefore less likely to tell the truth now.