Six years afterward appeared Galileo's treatise on mechanics.
To this great Italian is due the establishment of the three fundamental laws of dynamics, known as the Laws of Motion.
The consequences of the establishment of these laws were very important.
It had been supposed that continuous movements, such, for instance, as those of the celestial bodies, could only be maintained by a perpetual consumption and perpetual application of force, but the first of Galileo's laws declared that every body will persevere in its state of rest, or of uniform motion in a right line, until it is compelled to change that state by disturbing forces.

A clear perception of this fundamental principle is essential to a comprehension of the elementary facts of physical astronomy.

Since all the motions that we witness taking place on the surface of the earth soon come to an end, we are led to infer that rest is the natural condition of things.

We have made, then, a very great advance when we have become satisfied that a body is equally indifferent to rest as to motion, and that it equally perseveres in either state until disturbing forces are applied.

Such disturbing forces in the case of common movements are friction and the resistance of the air.