i really dont know how to respond to your statement that they respond to a stimulus and its obtuse.
pain is a stimulus
Hrmm, on a different argument about semantics you are arguing by division.
Pain is stimulus, but not all stimulus is pain.
Also, one thing and ONLY one thing feels pain. The brain. We interpret the nerve signals as pain in our brains. As pointed out, it is a cognitive function. Reacting to stimulus is not the same as everything "reacts" to stimulus. Take for example water. Apply some heat to water and you get steam, but apply some cold you get ice. In both cases the water reacted to a stimulus and responded. Can either response be associated with "pain" for the water? No.
It takes nerves and a cognitive function to determine pain. Want further proof? What happens when you are anesthetized, or something else prevents signals from nerves from reaching the cognitive part of your brain? You get no pain response. If I block the nerve center to your arm and then stab your hand you won't feel a thing. If I completely knock you out so your brain stops cognitive functioning, such as with general anesthesia then you won't feel anything done to your body at all.
The simple fact is that it take a stimulus input AND cognitive functions to feel "pain." Anything less than that is just the same scale as water reacting to temperature.