[wildmanx]

Depends on how narrow your definition of "pain" is.

Take a plant and put a sharp needle into its growth path. Many plats will avoid it and adjust their growth path, once they got into contact. Just like a human, a dog or a fish. The time scale is different, the effect is the same. It's a sensation that leads to avoidance of a certain negative stimulus.

[keiferski]

Correct. Pain just means “similar to human beings’ negative sensory input” which is very myopic, if you think about it.