in Philosophy, denotes the cause of the change in the state of a body, when, from being at rest, it begins to move, or has a motion which is either not uniform or not direct. Whilst a body remains in the same state, either of rest or of uniform and rectilinear motion, the cause of its remaining in such a state is in the nature of the body, and it cannot be said that any extrinsic force has acted on it. This internal cause or principle is called inertia. See Mechanics and Physics.