… is from page 350 of Karl Popper’s 1963 collection, Conjectures and Refutations: The Growth of Scientific Knowledge; specifically, it’s from Popper’s 1956 essay (originally a 1954 address to the Mont Pelerin Society) entitled “Public Opinion and Liberal Principles”:
The doctrine that public opinion is not irresponsible, but somehow ‘responsible to itself’ – in the sense that its mistakes will rebound upon the public who held the mistaken opinion – is another form of the collectivist myth of public opinion: the mistaken propaganda of one group of citizens may easily harm a very different group.
