Until the time of Newton, astronomy had been considered at best a theoretical science, based little on direct evidence. It was more often referred to as a philosophy, for its proponents were hypothesizers without proof, and they more often than not invoked the divine plan of god, or the existence of a universal soul in efforts to explain the structures and phenomena they observed. However, Newton's Principia changed the image of astronomy drastically. His theories were backed by solid mathematic tools, and his mechanical explanation of the universe contained no hint of mysticism or spirituality. With the advent of Newton, astronomy earned the right to be known as a science.