The former university student is charged with five counts of attempted murder after he attacked his tutor and classmates with a baseball bat during a statistics class at the ANU.

He does not deny the attack. But his lawyers have put at the centre of the case the question of Mr Ophel's mental impairment, a question the ACT Supreme Court jurors must consider.

The 20-year-old has pleaded not guilty by reason of mental impairment, and his lawyers argued his actions were those of someone suffering from psychosis.

They said the former student believed he was operating under pressure from aliens or higher beings.

The Crown contended the attack had involved a thought-out plan.

He had performed Google searches for ACT murder laws, ACT life sentences, "ISIS beheadings live links" and "how easy is it to kill someone with a baseball bat", it said.

In summing up the case, Justice John Burns told the jury the test for mental impairment was whether Mr Ophel could not control his conduct, or that he did not know it was wrong.

Mr Ophel would not know his conduct was wrong if he "could not reason with a moderate degree of sense and composure about whether the conduct, as seen by a reasonable person, is wrong".

Justice Burns said the Crown's best evidence of Mr Ophel's ability to reason his conduct was wrong was that he did reason it was wrong.

But he went on: "This is an example of the fallacy that has permeated the Crown case from the beginning."

The judge recounted details of a trial of a man in England some 120 years ago, who stood up in a playhouse and shot at King George III.

James Hadfield missed, but was apprehended and charged with treason.

The man was a soldier in the Napoleonic wars and had been struck in the head during battle, suffering a serious brain injury.

The man had delusions that he needed to die to save the world, the judge said.

But Hadfield, being a good Christian, could not kill himself, Justice Burns told the jury. He instead decided to commit a crime that would result in his execution.

He knew what he was doing was against the law, Justice Burns said. "That's why he was doing it."

The judge said the man was suffering delusions that impeded his reasoning, and 120 years ago was acquitted of treason on the grounds of insanity.

"And yet 120 years later we are met with the same argument, that what is important is the subjective mental state of the accused. It is not," he said.

Justice Burns said knowing conduct is against the law was not the same as being able to reason the conduct as seen by a reasonable person was wrong.

The judge also said experts had given evidence that the fact a person can plan a course of action is not inconsistent with mental impairment.

The jury retired to consider its verdict at about 11.00am on Thursday.

It will return to continue deliberating on Friday.