Clarke v R, 2012 CMAC 3 has a useful review of the law regarding credibility:
 Several basic principles inform my decision on this ground of appeal.
 First, witnesses are not "presumed to tell the truth". A trier of fact must assess the evidence of each witness, in light of the totality of the evidence adduced in the proceedings, unaided by any presumption, except perhaps the presumption of innocence: R. v Thain, 2009 ONCA 223, 243 CCC (3d) 230, at para 32.
 Second, a trier of fact is under no obligation to accept the evidence of any witness simply because it is not contradicted by the testimony of another witness or other evidence. The trier of fact may rely on reason, common sense and rationality to reject uncontradicted evidence: Aguilera v Canada (Minister of Citizenship and Immigration), 2008 FC 507, at para 39; R.K.L. v Canada (Minister of Citizenship and Immigration), 2003 FCT 116, at paras 9-11.
 Third, as juries in civil and criminal cases are routinely and necessarily instructed, a trier of fact may accept or reject, some, none or all of the evidence of any witness who testifies in the proceedings. Said in somewhat different terms, credibility is not an all or nothing proposition. Nor does it follow from a finding that a witness is credible that his or her testimony is reliable, much less capable of sustaining the burden of proof on a specific issue or as a whole.
 Fourth, reviewing courts should take a functional, context-specific approach to the adequacy of reasons. The reasons, in a case such as this, must be sufficient to fulfill their purpose of explaining why the person accused was convicted, providing accountability for the decision, and permitting effective appellate review. In considering the sufficiency of the reasons, we should read them as a whole, in the context of the evidence adduced and the arguments advanced at trial, and of the trial itself: R. v R.E.M., 2008 SCC 51,  3 SCR 3, at paras 15-16.
 Fifth, reasons fulfill their purposes if, read in context, they show why the judge made his or her decision. By reasons the judge tries to tell the partieswhat she or he has decided and why she or he made that decision. The reasons must reveal a logical connection between the "what" (the decision) and the "why" (the basis for the decision): R.E.M., at para. 17.
 Sixth, where a case turns largely on credibility, a reviewing court must consider the sufficiency of reasons in light of the deference afforded to trial judges on credibility findings. It is rare that deficiencies in the trial judge's credibility analysis, as expressed in the reasons for judgment, will merit intervention on appeal: R. v Dinardo, 2008 SCC 24,  1 SCR 788, at para 26. On the hand, a failure to sufficiently articulate how credibility concerns were resolved, or a legal error evident in their resolution may constitute reversible error:Dinardo, at para 26; R. v Braich, 2002 SCC 27,  1 SCR 903, at para 23: and R. v Gagnon, 2006 SCC 17,  1 SCR 621, at paras 20-21.