We all know how irritating philosophers can be. What would they make of this: “Expert evidence need not be opinion evidence”?
Most statements about the world outside our own bodies are opinions. We can only opine about the reality that casts the shadows on the wall of Plato’s cave. But law is not philosophy.
The sentence quoted above is from Lang v The Queen [2023] HCA 29, per Kiefel CJ and Gageler J at [5]. The case is partly about the sufficiency of proof of guilt, and partly about the admissibility of expert opinion evidence. It is on the latter point that Kiefel CJ and Gageler J wrote, “to elaborate on the common law principles” [4]. Perhaps these judges read the judgments of their colleagues (Gordon and Edelman JJ, who jointly dissented on the application of the law to the evidence in this case, and Jagot J who wrote separately agreeing with the conclusion of Kiefel CJ and Gageler J that the challenged evidence here was admissible) and decided that a bit more was needed.
Principles developed under Australia’s uniform evidence legislation apply also where the law on the admissibility of expert opinion evidence is common law [11]-13]: [1]
“Those principles require that, in order to satisfy the condition of admissibility that the opinion of an expert be demonstrated to be based on specialised knowledge or experience, the inference drawn by the expert which constitutes the opinion be supported by reasoning on the part of the expert sufficient to demonstrate that the opinion is the product of the application of the specialised knowledge of the expert to facts which the expert has observed or assumed. … it is enough that the opinion be demonstrated to be based substantially on that specialised knowledge … Reasoning sufficient to demonstrate that the opinion formed by an expert is the product of the application of his or her specialised knowledge need not be limited to formal induction or deduction. Speculation, however, is not reasoning … Nor is intuition. [The expert’s judgement] requires the justification of reasoned explanation when its conclusions are controverted.”
The utility or value of an opinion can be addressed in two separate ways, reflecting respectively admissibility and weight: the sufficiency of the demonstration that the opinion is the product of the application of specialised knowledge, and the extent to which the expert’s reasoning is clear and convincing. [15] On the admissibility question, lack of cogency (persuasiveness) is beside the point, [16] or at least it is unless a separate challenge to admissibility is made on the grounds that the probative value of the evidence is exceded by its illegitimately prejudicial effect [17]. Such a separate challenge was not made in this case [18], [25].
Here, the issue was whether the expert had drawn substantially on his specialised knowledge in engaging in the process of reasoning that led to the formation of his opinion.
The majority held that he had. His evidence was a process of inferential reasoning, throughout which he had compared what he had seen - a single stab wound with indicia of multiple thrusts and a turning of the blade in the wound - with what he had previously seen and read of the features of relevant but not identical cases involving suicide and assault [24], [469].
In dissent, Gordon and Edelman JJ held that the expert had not shown how his expertise provided a substantial basis for any connection between the facts and the opinion that he expressed [242]. Instead, the asserted connection was speculative [239].
A key difference in the interpretation of the expert’s evidence here is that while the dissenters attached significance to the witness’s description of the wound as “odd”, indicating that the witness was therefore forced to speculate, the majority understood the witness to have used the word “odd” in the sense of not fitting a standard pattern, and the witness was properly applying what he had previously seen and read about to what he observed [23].
_________________________________________
[1] We can expect the reverse to apply in New Zealand: principles developed at common law will inform the case law under the Evidence Act 2006, s 25.
