 This he did by introducing two new principles. In Day One of his Dialogue Concerning the Two Chief World Systems (1632), Galileo argues that matter will move naturally along circular trajectories, neither speeding up nor slowing down. Then, in Day Two, he introduces his version of the famous principle of the relativity of observed motion. This latter holds that observers cannot detect uniform motions they share with objects under observation; only differential motion can be seen. Of course, neither of these principles was entirely original with Galileo. They had predecessors. But no one needed them for the reasons that he did, namely that they were necessitated by a unified cosmological matter.