The law of inertia is the basis of the new physics of the seventeenth century. This law is also true according to modern physics. Galileo discovered the law during the first decade of the seventeenth century, but in fact he did not understand the law in the general way we have formulated it here. The general formulation of the law of inertia was devised by Galileo's pupils and byDescartes - a French philosopher, mathematician and physicist. This law is also the first of Newton's three laws.