Inertial frame of reference
Whether on a stationary boat (at the dock) or in straight and uniform movement, the cannonball dropped at the top of the mast falls to the same place at the bottom of the mast. Galileo concludes that for an observer on the boat, the observation of the fall of the cannonball does not allow him to distinguish whether he is in uniform rectilinear motion or at rest.
In a famous sentence, Galileo said "movement is like nothing". In other words, the study of movement is intimately linked to the frame of reference in which it is studied, which is summarized by this statement: Movement is relative.
This statement is one of the foundations of all the physics we know, since it lays the foundations of kinematics (study of motion, velocity addition formula, principle of inertia, etc.).
It also defines two main categories of reference frames: Inertial referential frames, for which the same laws of physics apply, and non-inertial referential frames for which inertial forces must be used.
This principle of relativity completely denies Aristotle’s principles according to which movement and rest are two completely different concepts.