The term "stability" in this context is a bit misleading and tends to confuse.
If you consider the standard diagram of an aeroplane - CG at the front, main lifting surface a little behind that, and a tailplane rather further behind that providing a downforce - you have a system in balance (hopefully!).
If you move the CG back to co-incide with the centre of pressure (CP) then the tailplane has no work to do. So, any movement of the elevator will have a very strong effect. This is referred to as neutral stability and the aeroplane will be virtually impossible to maintain at the desired pitch attitude or airspeed.
As you move the CG forward, the tailplane starts having to do some work. This means that the elevator is just modifying the amount of downforce from the elevator, and so you have a sensible amount of control over the aeroplane. The aft CG limit is set at the point where the company test pilots decided that the aeroplane was safely controllable by the average pilot.
Keep moving the CG forward and the elevator is doing more work. This means that the effect of elevator movement reduces -it needs to deflect more for the same effect. Test Pilots will measure this as either stick displacement per airspeed change, or (much more importantly since we fly by feel, not stick position) stick force per airspeed change.
Eventually you'll reach a point where it doesn't matter how much you move the stick you don't have enough control over the aeroplane - the most common way this'll exhibit itself is by the aeroplane refusing to pitch up into the flare properly at the approved approach speed. Again, the company test pilots will set the forward CG limit at the point where they consider that the aeroplane is just safely controllable by an average pilot. (Although occasionally you hit a tailplane structural limit first - the tail is working very hard at forward CG).
But what's important to realise is that what you are told is "pitch stability", is what a flight tester will more formally call "apparent longitudinal static stability" and is really just the amount you have to push or pull the stick to get a given airspeed or attitude change.
Hope that helps a bit,
G
N.B. If you like hard sums, I can recommend a couple of good textbooks, but frankly they are best avoided until you've got the basic mental model fixed anyhow. If you have, then most people don't need the maths!