The jury in the Vicky Pryce trial did more than fail to reach a verdict after some 15 hours of deliberation. It asked a series of questions that must have damaged public confidence in the entire system of trial by jury.

One question was so extraordinary that it suggested the jury had completely failed to understand what it was there for. It was: "Can a juror come to a verdict based on a reason that was not presented in court and has no facts or evidence to support it, either from the prosecution or defence?"

Betraying not the slightest hint of irritation, Mr Justice Sweeney told the jury that the answer was no. It was, of course, completely contrary to the jury members' oath for them to decide the case on anything other than the evidence.

Another question, almost as bad, was: "Can we speculate about the events at the time Vicky Pryce signed the form [accepting her husband's penalty points] or what was in her mind at that time?"

No to that one too, was the judge's response. The drawing of inferences is a permissible process, he explained; speculation is not.

Another alarming question was: "In the scenario where the defendant may be guilty, but there is not enough evidence provided by the prosecution at the material time … to feel sure beyond reasonable doubt, what should the verdict be? Not guilty or unable/unsafe to provide a verdict?"

On some of the questions, the judge simply batted the question back to the jury.

"Can you define what is reasonable doubt," they asked. "A reasonable doubt is a doubt which is reasonable," the judge replied, explaining that the law did not allow him to go further.

The jury also asked: "Does the defendant have an obligation to present a defence?" The judge reminded them he had told them, in terms and in writing, that the defendant did not have to prove anything at all.

These were among the eight questions sent in by the jury foreman, presumably on behalf of several jurors. One further question, from an individual juror, was utterly speculative. It was: "Would religious conviction be a good enough reason for a wife feeling that she had no choice, ie she promised to obey her husband in her wedding vows and he had ordered her to do something and she felt she had to obey?"

Sweeney pointed out the question had nothing to do with this case. Pryce had never suggested any such reasoning was behind her decision to take Huhne's points.

What's particularly alarming about all this was that it was a short case.

The judge had provided jurors with a copy of his directions and asked jurors two simple questions. If the answer to either of them was yes, the defendant was guilty. If the answer to both of them was no, it was not guilty.

Sweeney addressed the jury firmly and clearly, in language that could not be faulted by prosecution or defence lawyers and that is unlikely to be questioned by the court of appeal.

What is clear, though, is that the jury did not understand what he was saying, even given an opportunity to read it back afterwards. The only inference one can draw from that was the judge's level of spoken and written English was higher than the level that the jury could comprehend; he went over their heads.

How can we avoid such problems happening in future trials? There may be scope for training judges to speak in short, simple sentences, but some questions of law do not lend themselves to oversimplification.

There may be scope for testing jurors' comprehension abilities before allowing them to sit. Jurors who were unable to read the oath – "I have forgotten my glasses" was the usual excuse – used to be stood down. But any wider test of intelligence would be seen as invidious and impractical.

So it may be time to consider whether we are right to entrust the most serious criminal cases to the hands of unqualified lay people. Lawyers such as Sir Louis Blom-Cooper QC have argued for years that we would be better off if more trials were decided by qualified judges, who would have to give reasons for their decisions.

It would be wrong to change our legal system just because one London jury appeared to be utterly at sea. But if we start to fall out of love with juries in the future, this case may be seen as the beginning of the end.