… is from page 168 of the 5th edition (1966) of Karl Popper’s 1945 study, The Open Society and Its Enemies (footnote excluded):
Aestheticism and radicalism must lead us to jettison reason, and to replace it by a desperate hope for political miracles. This irrational attitude which springs from intoxication with dreams of a beautiful world is what I call Romanticism. It may seek its heavenly city in the past or in the future; it may preach ‘back to nature’ or ‘forward to a world of love and beauty’; but its appeal is always to our emotions rather than to reason. Even with the best intentions of making heaven on earth it only succeeds in making it a hell – that hell which man alone prepares for his fellow-men.