The problem which lies at the root of theories of historical continuity and discontinuity is the meaning of a "turning point" in history, whether it be Caesar's crossing of the Rubicon, the Fall of the Bastille, or the publication of Newton's Principia. The theory of continuity ideally asserts that historical change proceeds gradually by small steps, and thus denies the existence of any sudden, large-scale changes or revolutions; it therefore either denies that there exist abrupt tuming points in history or asserts that they are merely convenient landmarks. There is no actual location called the equator; it is a landmark only because geographers have made it so. The approach to the equator is utterly continuous; as a landmark, the equator is a discrete, hypothetical line.