I am pretty sure it originates with S. T. Coleridge, but will track down the source as well as more echoes in comments (eventually). The quotation, or rather the idea behind it, made the alliance between the Liberals among the Anglican Churchmen, who held a monopoly of power at Oxford and Cambridge as late as 1870, and the radicals from London who demanded the disestablishment of the Church and the democratization of the country. In other words, the quotation allowed the coming together in the practical business of transforming England's ancient fortresses of the Anglican Reformation into modern universities, which is to say that it allowed conversation between parties who fundamentally disagreed on certain vital beliefs.
In the 1870s in Cambridge, which I studied long ago for a PhD, the quotation became even a principle of research. This was to impose a dialectic on the opposition of opinons, such that identifying the disagreement became a method of reaching to a new synthesis. Echoes of this dialectic are (imo) discerned in Tolkien's 1936 allegory of 'Beowulf' as a tower that the builder's friends destroy and his relatives don't get (the friends and relatives see, respectively, different parts of the picture - and the two parts need to be put together if we are ourselves to climb the stairs and see the sea).
Anyway, I need to dig up my old research to get to the source, but here from a quick google are two versions of the original quotation.
In all intellectual debates, both sides tend to be correct in what they affirm, and wrong in what they deny.
John Stuart Mill
Men are generally right in what they affirm and wrong in what they deny. What we deny is generally something that lies outside our experience, and about which we can therefore say nothing.
H. Richard Niebuhr
More to follow...
