I think whether something is painful or not depends on the perspective. Humans can voluntarily suffer lots of pain for a good cause. And vice versa, small thing

I think whether something is painful or not depends on the perspective. Humans can voluntarily suffer lots of pain for a good cause. And vice versa, small things can be extremely painful if we don't understand them. (Edit: In particular, to some people, even prospect of loss of their own property is more painful than death of other people.) So it comes down to your expectations. In reinforcement learning, this is very clear - bad predictions of your future actions lead to negative reward, i.e. pain. We could almost say pain depends on your expectations of reality (however the expectations are not always consciously expressed). So you can torture a human, because our nerves have learned (have the expectation) of not being in pain as a normal state. If they could learn more (like RL agent), they would eventually learn that the pain is just nature of the world, and ignore it. Similarly, Data can ignore pain of his missing hand because he trusts the Enterprise crew to put it back. If they didn't and betray him, or if he hurt his arm so badly it couldn't be replaced, then I think, on some level, he would feel pain. (Edit: Data mourned death of Tasha Yar, this can be interpreted as pain.)