What do you mean by inertia or "state of inertia" exactly ?
But yes, every mental-illness are not the same because of their variety and the difference among the people who experience those illnesses. That's why doctors talk about a spectrum.
- A person with no inrtia is someone like me, no motivation and stuck at home with no motivation, in the incapacity of doing anything relevant beside surfing on the internet.
- A person with inertia is someone like Churchill, perhaps a lack of motivation but in a state of life where the environment, the social interactions, the job, the hobby acts as a motor for the person and therefore create enough motivation to keep them going, sometime without any help.
"Inertia" is exactly that, a state where you are already put into motion and you keep going because there is enough "push" around you to do that.
Sadly when you are not in that state, you can only hope that a treatment and someone will help to push you forward.
Let's take the example of a boat sailing at sea:
- Having inertia means that you can lack wind (motivation) but still be able to move
- Not having inertia means that there is no wind and the boat is not into motion meaning that you can't go forward.
The entire point of healing is to give back the inertia to the patient.