If a defendant has no personal knowledge of the events, or of a reason that a witness may be lying or mistaken, can he provide evidence that contradicts his own testimony?

"If a defendant has no relevant personal knowledge of the events, or of a reason that a witness may be lying or mistaken, he might have no relevant testimony to provide. No witness may give testimony based on conjecture or speculation. [Citation.] Such evidence is irrelevant because it has no tendency in reason to resolve questions in dispute. [Citation.]" (People v. Chatman, supra, 38 Cal.4th at p. 382.) However, "[a] defendant who is a percipient witness to the events at issue has personal knowledge whether other witnesses who describe those events are testifying truthfully and accurately. As a result, he might also be able to provide insight on whether witnesses whose testimony differs from his own are intentionally lying or are merely mistaken." (Ibid.) That is particularly true if a defendant knows these witnesses and knows of reasons why they might lie. (Ibid.)