In the text “The Cause of Motion”, I argue the following 6 flaws in the definition of inertia and the law of inertia and propose a solution to correct these problems and mechanism for inertial motion. 1. The definition of inertia does not answer the cause why the object has inertia. 2. The law of inertia does not answer what is the cause of motion. 3. Stationariness and rectilinear motion at constant velocity are two effects that need two causes to explain. 4. We have not found the elementary particles or properties responsible for inertia in matter. 5. The inertial motion of the same matter in gaseous, liquid, solid or other states is completely different. 6. No experiment has proved the existence of inertia or the law of inertia.