Karl Marx and Friedrich Engels emerged as socialist theoreticians of the first rank at a time when many of their rivals were still engaged in efforts to design visionary ideal societies. Although they did not hesitate to appropriate key concepts from some of the most original of these architects of the future, Marx and Engels were quite rightly convinced that one of their first tasks must be to still the siren song of the so-called utopian socialists if they hoped to build a politically effective movement. They therefore launched a polemic against the “systematic pendantry” and the “fanatical and superstitious belief[s]” of these builders of “castles in the air” which eventually proved so effective that many contemporary as well as later commentators denied, ignored, or at the very least vastly underestimated the significance of the millenarian appeal of Marxism itself.