What is the main value in a free press? To hear the media tell it, as in many of the 350-plus editorials published in co-ordination last week in response to Donald Trump’s anti-press rhetoric, the answer is factual, objective coverage of events.

But that is not what the framers of the Constitution thought, or what Justice Oliver Wendell Holmes had in mind when he crafted modern free-press jurisprudence during the First World War. It also does not match how most newspaper writers thought of themselves until the emergence of journalism as a “profession” in the postwar period.

The classic reason for valuing a free press is that it expresses a variety of opinions, especially those that differ from the Government. That in turn fuels democracy, which requires disagreement with those in office so that the public can consider choosing new leaders instead.

It is crucial to keep in mind the value of opinion in an era when editorials and opinion journalism are being constantly questioned. The President accuses the media of being “the opposition party”, implying a failure of objectivity. But it’s not a bad thing for opinion journalism, including the editorial boards of leading newspapers, to see themselves as the opposition to Trump. Opposition such as that keeps democracy alive, and constitutes one of the core responsibilities of a free press.

To be clear, I’m not speaking against the journalistic aspiration to discover and report facts. There is a difference between factual truth and lies. Because no one else in society seems to have the time or the interest to police that boundary, the media should try their best to do so.

Rather, I’m pointing out that the need for fact-checking was not the basis for the traditional view that a free press is necessary for democracy.

Consider James Madison, the author of the First Amendment. He gave press freedom little thought when he was actually drafting the amendment, coming to focus on it only when partisan battles between his Republican Party and Alexander Hamilton’s Federalists began to define national politics.

Hamilton and his supporters used a newspaper called the Gazette of the United States to promote Federalist views, even publishing anonymous essays by John Adams, then serving as Vice-President. Madison, with encouragement and help from Thomas Jefferson, who was then Secretary of State, facilitated the creation of a competing newspaper, the National Gazette, to combat Hamilton — by offering a Republican perspective.

The duelling newspapers existed to present duelling opinions. No one on either side thought that either newspaper was objective. Indeed, it is doubtful whether the idea of factual objectivity as a journalistic goal was even imagined in 18th-century America.

When, as president, Adams signed the Sedition Act passed by the Federalist Congress, he used it to prosecute Republican newspaper editors. Many were convicted, imprisoned and fined for insulting the Adams Administration.

Madison sprang into action, composing a report adopted by the Virginia legislature that contained the first detailed defence of the First Amendment. He argued that a free press was necessary to ensure free elections because only a free press could provide the people with the ideas they needed for “the just exercise of their electoral rights”. This was a defence of opinion journalism — exactly what the Sedition Act punished.

More than a century later, when the Supreme Court first articulated the value of a free press and free speech, Holmes famously wrote that the First Amendment should be understood in terms of the metaphor of a marketplace of ideas. “The best test of truth,” he wrote, “is the power of the thought to get itself accepted in the competition of the market ... That, at any rate, is the theory of our Constitution.”

Holmes left no doubt that he was thinking about opinions, not objective facts. He said: “We should be eternally vigilant against attempts to check the expression of opinions that we loathe and believe to be fraught with death, unless they so imminently threaten immediate interference with the lawful and pressing purposes of the law that an immediate check is required to save the country.”

Holmes’s whole theory rested on his philosophical pragmatism. Opinions were provisional, not objectively provable. Even the very idea of the freedom of speech, he said, “is an experiment, as all life is an experiment”.

The upshot is that we need to remember that a free press preserves democracy mostly by allowing for the expression of alternative points of view. An autocratic or dictatorial government seeks to suppress differences of opinion just as much, or more, than it seeks to suppress facts.

The co-ordinated editorials last week are a case in point. They are expressions of opinion, not fact. By airing those opinions, the newspapers created a news cycle of their own — one focused on the media itself, rather than Trump’s criticisms of it.

It is great that newspapers try to be objective. But the human limitations of editors and writers mean this can only ever be an aspiration, not a fully accomplished reality.

That’s OK. The justification for a free press does not depend on it being objective. To the contrary, a free press is necessary for democracy because it preserves the multiplicity of opinions and points of view.

• Noah Feldman is a Bloomberg View columnist. He is a professor of constitutional and international law at Harvard University and was a clerk to US Supreme Court Justice David Souter. His seven books include The Three Lives of James Madison: Genius, Partisan, President and Cool War: The Future of Global Competition