You want juries to decide justice? I want them to decide guilt or innocence; I'm quite happy to leave justice for the sky-daddy.
That really is a false dichotomy, The Atheist. In the vast majority of cases the jury does exactly what you say and anecdotal evidence suggests that most take their responsibilities very seriously: certainly those I know who have been called do: and those who have posted about their own experience tend to confirm this: I have seen nothing to suggest they are frivolous nor that they are incapable of discharging that responsibility on the basis of the evidence, though there have been some worries about their ability to do this in complex financial cases. (as an aside I am quite interested that there was a strong push to take such cases out of the jury system on those grounds: since there now seems to be ample evidence that professionals were incapable of understanding complex financial arrangements too

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Nonetheless I see a need for a human element within the system for the reasons I outlined above. Your response to the Clive Ponting case was to say it was not an argument in favour of juries but an argument in favour of changing a bad law. I think you miss the point. It was the view of the jury in that case that it was a bad law and they refused to convict on that basis: though Ponting was clearly guilty. That is a safeguard which I do not think could have come from an entirely professional system.
If there was no such safeguard how do you suggest a bad law would be identified as such? Most people are not aware of the legal position on many issues: professionals within the legal system are very conscious that it is not for them to make law but to apply it (some will make representations on particular issues, perhaps: but there are VERY strong inhibitions about that and enormous resistance to interference in politics by the judiciary for very good reason): That is how they are trained to see it and so they should be
So one way of identifying when the law is bad or has become out of step with the view of the people is a situation like Pontings: the jury did not believe the law was just: and refused to convict. That was a very controversial matter and it led to a big debate. It also led to some changes in the law and I do not see another mechanism which would have had that result.
It is not perfect because the jury for the most part accepts that their role is to determine guilt on the basis of the law as it stands: it is probably true that Ponting got the result he did partly on irrelevant matters: he was a senior civil servant and he dressed and talked well. But I do not believe the decision was made on those factors, though they probably played a part: it was primarily based on the defence of public interest which he mounted: his advantages probably let him present his case better, but it was still the case he made which carried the jury: and this does not happen very often.
If you wish to remove this safeguard: which is indeed based on the jury's role in giving some weight to justice rather than the letter of the law, how do you propose to replace this function?