The issue whether or in which sense science informs us about the real world has pervaded the history of thought since antiquity. Ancient and Renaissance astronomers alike extensively addressed the question if astronomical theory was restricted to 'saving the phenomena" of celestial motion or could single out the real mechanisms that produced the stellar appearances. The relationship between knowledge and the world has been systematically discussed since the middle of the 19^th century. The issue has always been whether what science tells about the world is determined unambiguously by the facts or whether the content of scientific theory is in some deep-reaching sense dependent on the human condition.