How Strong Must Circumstantial Evidence Be For Consideration By a Court?
Circumstantial Evidence Should Be Assessed With An Open Mind to Plausible Theories Which May Be Inferred From the Evidence. Accordingly, Whether the Circumstantial Evidence Works For or Against a Person Requires a Review of Reasonable Possible Interpretations For the Evidence.
Understanding That Assessing Circumstantial Evidence Involves Review of Various Possible Theories
A legal case may involve civil law issues or criminal law issues, among other types of issues, for which the review and assessment of circumstantial evidence has a bearing upon the case. Where an important legal case may be decided upon the strength, or weakness, of circumstantial evidence, it is especially important that the review and assessment of the circumstantial evidence be done very carefully and a person should be deemed guilty, or as having acted wrongfully, only where a reasonably alternative interpretation is unreasonable.
The difference between what is circumstantial evidence versus what is direct evidence was provided by the Supreme Court of Canada within the case of R. v. Villaroman,  1 S.C.R. 1000 where it was said:
 An explanation of the difference between direct and circumstantial evidence is included in most criminal jury charges and rarely causes difficulty. One example of how this distinction may be conveyed to a jury is found in s. 10.2 of the Model Jury Instructions (online) prepared by the National Committee on Jury Instructions of the Canadian Judicial Council:
 As I explained at the beginning of the trial, you may rely on direct evidence and on circumstantial evidence in reaching your verdict. Let me remind you what these terms mean.
 Usually, witnesses tell us what they personally saw or heard. For example, a witness might say that he or she saw it raining outside. That is called direct evidence.
 Sometimes, however, witnesses say things from which you are asked to draw certain inferences. For example, a witness might say that he or she had seen someone enter the courthouse lobby wearing a raincoat and carrying an umbrella, both dripping wet. If you believed that witness, you might infer that it was raining outside, even though the evidence was indirect. Indirect evidence is sometimes called circumstantial evidence.
Rule, inherent to definition
In R. v. Khan, 2019 ONCA 81, the Court of Appeal, while citing the Supreme Court explanation in Villaroman, reminded of the requirement to review whether circumstantial evidence can give rise to reasonable possibilities other than guilt by stating:
When assessing circumstantial evidence the trier of fact should consider “other plausible theories" and "other reasonable possibilities” which are inconsistent with guilt.
The reminder in Khan followed age old and well established principles that circumstantial evidence may be applied for a finding of guilt where rational inference supports only the presumption of guilt. These principles were explained well within the Mezzo v. R.,  1 S.C.R. 802 decision of the Supreme Court as well, decades earlier, by the Court of Appeal for Alberta in the case of R. v. McEwan, 1932 CanLII 308 wherein each it was respectively stated:
12. A similar question as to the scope of the trial judge's function on a motion for a directed verdict arose in the context of circumstantial evidence in R. v. Comba, supra. In that case this Court set aside a jury verdict of guilt. Sir Lyman Duff C.J. stated, in effect, that cases comprised wholly of circumstantial evidence should be measured against the rule in Hodge’s Case (1838), 1838 CanLII 1 (FOREP), 2 Lewin 227, 168 E.R. 1136. The rule states that where all the evidence is circumstantial the accused can be found guilty only if the evidence is both consistent with guilt and inconsistent with any other rational conclusion. He also remarked that this would have been an appropriate case to direct a verdict of acquittal. A line of jurisprudence subsequently developed which incorporated the rule in Hodge’s Case into the trial judge's deliberations on a motion for a directed verdict, thus creating an exception where circumstantial evidence was concerned to the narrow role of the judge at this stage of the trial.
Proof by circumstantial evidence being a matter of logical reasoning from facts admitted or established in evidence there is always the danger of the tribunal of fact, whether it be Judge or jury, jumping to conclusions from certain facts without due regard to other facts which are inconsistent with the hypothesis which the first set of facts seems to point to....
There being no direct evidence, the case must rest, on the circumstantial evidence alone, and the general rule is that to amount to proof such evidence must be not merely consistent with guilt but inconsistent with innocence.
Proper Review Criteria
The assessment of circumstantial evidence requires more than just the evaluation of whether the circumstantial evidence rationally enables the inference that a possibility of guilt exists, when the circumstantial evidence is assessed such requires the review of other possibilities as per Villaroman where it was said:
Trial by Jury, instructions for deliberating
Where a criminal case is proceeding before a jury, where the assessment of circumstantial evidence is involved, the Trial judge is without requirement to provide the jury with "special instruction" for the proper assessment of the circumstantial evidence per R. v. Griffin,  2 S.C.R. 42 which says:
 We have long departed from any legal requirement for a “special instruction” on circumstantial evidence, even where the issue is one of identification: R. v. Cooper, 1977 CanLII 11 (SCC),  1 S.C.R. 860. The essential component of an instruction on circumstantial evidence is to instill in the jury that in order to convict, they must be satisfied beyond a reasonable doubt that the only rational inference that can be drawn from the circumstantial evidence is that the accused is guilty. Imparting the necessary message to the jury may be achieved in different ways: R. v. Fleet (1997), 1997 CanLII 867 (ON CA), 120 C.C.C. (3d) 457 (Ont. C.A.), at para. 20. See also R. v. Guiboche, 2004 MBCA 16 (CanLII), 183 C.C.C. (3d) 361, at paras. 108-10; R. v. Tombran (2000), 2000 CanLII 2688 (ON CA), 142 C.C.C. (3d) 380 (Ont. C.A.), at para. 29.
 There is no question that the instructions in the present case fulfilled this essential requirement. The trial judge repeatedly made clear to the jury that a guilty verdict can only be rendered if guilt is the sole rational inference to be drawn from the circumstantial evidence. The issue to be resolved, rather, is whether the use of the words “equally rational” and “as reasonable” detracted from the otherwise correct instructions by leading jurors to believe, as the respondents contend, that they could only acquit if they found an innocent inference from the circumstantial evidence of the same weight as an inference of guilt. It is argued that the impugned language had the potential to engage the jury in an abstract comparative exercise, assessing the qualitative reasonableness of one inference against another when the mere existence of any rational, non-guilty inference is sufficient to raise a reasonable doubt.
 Read in context, as all jury instructions must be, it becomes apparent that the words “equally rational” and “as reasonable” were not used as measures of the comparative value or weight of a non-guilty inference, but, as the judge himself explained to counsel, to describe the quality of any inference, guilty or otherwise, which might be drawn from the circumstantial evidence. Indeed, no one argues that the jury should be entitled to base its decision on irrational or unreasonable inferences. When the instructions on circumstantial evidence are read as a whole, it is more likely than not that they would have been understood by the jury in the manner intended by the trial judge. That said, it is my view that the instructions on this point were not as clear as they could have been and, consequently, the trial judge was wise to recharge the jury as he did.
When circumstantial evidence is reviewed, assessed, and considered, by a judge or adjudicator, or when a judge is providing instructions to a jury on how to assess circumstantial evidence, a key principle involves the review of various plausible theories and reasonable possibilities to which the circumstantial evidence could be inferred.