In 1765, Joseph Priestley created what may be the world's first modern timeline, A Chart of Biography. This paper offers the first study of the philosophy underlying Priestley's timeline. It argues that Priestley was pushed towards representing times as lines by his views on abstract ideas and time, and there is no reason to believe that Newtonian absolutism grounds his uniform depiction of time. Further, the Chart confirms, and even advances, Priestley's views on human progress. Finally, this study shows that Priestley's conception of time as a line comprises a landmark contribution to the history of space-time parallelism.