If a trial judge's finding of credibility is to depend solely on which person he thinks made the better appearance of sincerity in the witness box, what is the test for credibility?

If a trial Judge’s finding of credibility is to depend solely on which person he thinks made the better appearance of sincerity in the witness box, we are left with a purely arbitrary finding and justice would then depend upon the best actors in the witness box. On reflection it becomes almost axiomatic that the appearance of telling the truth is but one of the elements that enter into the credibility of the evidence of a witness. Opportunities for knowledge, powers of observation, judgment and memory, ability to describe clearly what he has seen and heard, as well as other factors, combine to produce what is called credibility, and cf. Raymond v. Bosanquet (1919), 1919 CanLII 11 (SCC), 50 D.L.R. 560 at p. 566, 59 S.C.R. 452 at p. 460, 17 O.W.N. 295. A witness by his manner may create a very unfavourable impression of his truthfulness upon the trial Judge, and yet the surrounding circumstances in the case may point decisively to the conclusion that he is actually telling the truth. I am not referring to the comparatively infrequent cases in which a witness is caught in a clumsy lie.