From the last page, we have the idea of mass, which is something that cannot be separated from everyday objects. Since mass and objects can't be separated, if the mass of something moves, that something has to move with it.
In his First Law of Motion, Newton broke with common experience and hundreds of years of conventional thought by saying that, unless acted upon by other forces, mass will moving if it is already in motion, in a straight line, forever. He was able to make this fantastic break from normal thinking because of his ideal environment, which does not have the common resistive forces that slow down moving objects on Earth.
Before Newton, people always thought that moving things will eventually slow down to a stop naturally, since that can be seen everyday on Earth. What they didn't notice is that it is because of resistive forces like friction and air resistance that cause the stopping, not nature itself. Newton came to this conclusion more or less because of his study of stars and planets, since they move in space, which is practically without those resistive forces that we are familiar with.
Because of this law, we can also explain why it is difficult to start or stop moving something, or move things faster or slower, or even simply to change the direction of moving things. This difficulty is known as inertia, and therefore we also call this law as the Law of Inertia.