What is the test for credibility in a defamation case?

In that decision, itself a defamation case, the court provided at 356-57 the following guidance with respect to the assessing of 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. The credibility of interested witness, particularly in cases of conflict of evidence, cannot be gauged solely by the test of whether the personal demeanour of the particular witness carried conviction of the truth. The test must reasonably subject his story to an examination of its consistency with the probabilities that surround the currently existing conditions. In short, the real test of the truth of the story of a witness in such a case must be its harmony with the preponderance of the probabilities which a practical and informed person would readily recognize as reasonable in that place and in those conditions.