Abstract
Mechanics was the foundation of physics in the 19th century. Its own foundation was, however, problematic. Mach proposed a new foundation of mechanics. As we shall see, the cause of acceleration is no longer the force, which was the great problem at that time, but rather the mass. The definition of force, which is a reading of the equation of force, did not solve, however, the conceptual problem. If we want to know what force is and we require that force is F in the equation F=ma, then we have to analyze F and not define it through the other side of the equation, as it has been done. This analysis enables us to conclude that force stems from a mathematical manipulation; it is not something that we can find in nature. This conclusion, in turn, enables us to understand those authors who looked for force in nature and did not find it, as well as those who said that force is a mathematical fiction, a mere human concept, etc.