the mass media are often seen as fulfilling the vitally important rôle of fourth estate, the guardians of democracy, defenders of the public interest.

The term fourth estate is frequently attributed to the nineteenth century historian Carlyle, though he himself seems to have attributed it to Edmund Burke:

Burke said there were Three Estates in Parliament; but, in the Reporters' Gallery yonder, there sat a Fourth Estate more important than they all. It is not a figure of speech, or a witty saying; it is a literal fact, .... Printing, which comes necessarily out of Writing, I say often, is equivalent to Democracy: invent Writing, Democracy is inevitable. ..... Whoever can speak, speaking now to the whole nation, becomes a power, a branch of government, with inalienable weight in law-making, in all acts of authority. It matters not what rank he has, what revenues or garnitures: the requisite thing is that he have a tongue which others will listen to; this and nothing more is requisite.

Carlyle (1905) pp.349-350

Carlyle here was describing the newly found power of the man of letters, and, by extension, the newspaper reporter. In his account, it seems that the press are a new fourth estate added to the three existing estates (as they were conceived of at the time) running the country: priesthood, aristocracy and commons. Other modern commentators seem to interpret the term fourth estate as meaning the fourth 'power' which checks and counterbalances the three state 'powers' of executive, legislature and judiciary.