The Supreme Court is deliberating the legality of the advice Johnson gave Queen Elizabeth II to suspend parliament for five weeks until October 14, after two lower courts made conflicting rulings.
Johnson, who took office in July, says he suspended parliament to open a new session with a fresh legislative agenda.
His opponents claim he did so to silence opposition to his plans to take Britain out of the EU on time, with or without a divorce deal with Brussels.
Eadie said any decision to prorogue parliament was "inherently and fundamentally political in nature" as it concerned managing the government's legislative agenda, curtailing parliamentary debate and deciding on when it might be appropriate to end a sitting.
"This is, we submit, therefore the territory of political judgement, not legal standards," he said.
"If the basic attack is this was improper purpose, motivation or effect, because it was designed to stymie parliament, we respectfully submit: that is unsustainable."
He said parliament had previously passed laws on prorogation, but there was no law relevant to the case and so it was not for the court to decide upon.
The maximum 11 of the 12 Supreme Court judges are hearing the case, which began on Tuesday and will end on Thursday, although a decision may be deferred.
Judge Nicholas Wilson questioned Eadie on the lack of a witness statement from the prime minister.
"Isn't it odd that nobody has signed a witness statement to say 'this is true, these are the true reasons for what was done'?" he asked.
Eadie said cabinet documents had been produced which showed the aim of prorogation was to bring forward a new legislative agenda.
The Supreme Court is hearing appeals against two conflicting lower court decisions.
Scotland's highest civil court called the suspension of parliament "unlawful", but the High Court in England said it was not a matter for judges.
The Supreme Court must rule on whether they even have the right to adjudicate on the politically contentious issue, before considering Johnson's motivations.
Unlike Tuesday's chaotic scenes, there were few protesters outside the court on Wednesday.