Philosophy and theology are the main topics of this encyclical. But it extends further into the realm of culture and science. The encyclical is a document with firm distinctions between right and wrong, good and bad. Pius XII is convinced about the indivisibility and timeless nature of truth. The encyclical is flexible in all areas of scientific research which do not intrude into or exclude theology. It demands respect for the intellectual achievements of past generations, which were equally intelligent, but is not afraid to face a future with new questions and improvements. "Humani generis" generated much discussion at its time. It reflects many conservative positions of the Pope, but also his openness to science and new developments. It reflects his belief: "It is the primary duty of a Christian, to convince those who consider themselves modern, that human nature should not be interpreted with systematic pessimism nor with shallow optimism."