structural:reformatting, structural:summarization
Competency

The witness was unable to sense what he claimed to have (such as he could not see from where he was), or he lacked the requisite mental capacity. Older common law would exclude an incompetent witness from testifying. Modern rules, such as the Federal Rules of Evidence, allow the witness on the stand (in most cases) to consider competence as one of many factors that juries are to consider to determine credibility of the witness.