The sorcerer’s apprentice or The Red Shoes both turn on the idea that self-motion or self-initiative is just as much the ability to stop a process as to begin it; or that causing rest is just as essential to agency as causing action.
In light of this inertia is the absence of self-motion/ agency in physical beings. You hit a golf ball in space like the apprentice sets the pail in motion to get water: both never cease not because of they have some infinite reservoir of action or agency but because of the inability of either one to control itself or cause its action. The pail and the golf ball both move forever only because they are being moved forever and there is nothing they can do about it. They never stop moving precisely because they are not agents. For our own part, that we can’t control many processes we start in motion is true for the same reason it is for the pail or the golf ball, namely, the absence of knowledge which alone constitutes a thing as an agent in itself. Things self-cause as they are knowers: what does not know at all is caused by another, what knows imperfectly is imperfectly a cause of another, and what knows perfectly is perfectly the cause of another. Knowledge as self-agency, even as sensation, is immateriality as opposed to the physicality properly studied in natural sciences. Physical : created immaterial : uncreated immaterial :: pail : apprentice : Sorcerer.
What is true about inertia is also true about other physical processes as physical, sc. radioactive decay, the motion of fields, the orbitals of electrons, or any form of a conserved quantity.