The document discusses how our understanding of inertia and the first law of motion has evolved over time. Originally, Newton viewed inertia as the mechanism behind the first law, but now inertia is understood to be the phenomenon described by the first law. The first law states that objects at rest will stay at rest, and objects in motion will stay in motion with the same speed and direction, unless acted on by an unbalanced force. Examples are provided to illustrate the first law in action.