How do we know that plants and similar don't feel pain? As far as I see it plants just don't act like they're in pain, but that doesn't mean they aren't. They could just be very stoic about it.

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You might just as easily wonder how you know that other people do feel pain. Both questions are instances of what philosophers often call "the problem of other minds." I believe that other people have psychological states (thoughts, beliefs, sensations, etc.) and that things like rocks and plants do not. How do I know this? It seems on the face of it that I cannot know that someone (other than myself) is in pain in the same way that I can know that the table in front of me is brown; I cannot directly observe the pain of another person. So presumably I must know that someone is in pain on the basis of something I can observe: his or her behavior. I see a batter get hit by a pitch. He falls to the ground, grimaces and writhes around. From this behavior, I infer that the batter is in pain. In a similar vein, when a plant does not exhibit any of that kind of behavior (or anything that could be understood as behavior at all), I conclude that the plant does not feel pain (or anything else, for that matter). The problem is what justifies these kinds of inferences.

Some philosophers think that these inferences are justified because of what we have observed in our own case: I find that whenever I feel intense pain, I behave in characteristic ways. Then when I observe similar behavior in others, I assume that their behavior is caused by similar psychological states. Some worry, however, that the correlations between psychological states and behavior that I observe in my own case are not enough to ground inferences to cases other than my own. To use an example from Hilary Putnam: suppose I see a mole under my right arm. I've never had the chance to look at under anyone else's arm. It would seem rash to infer from my own case that everyone has a mole under his or her right arm. Good inductive inferences seem to require observing correlations in a wide variety of circumstances. But here, I have observed correlations in only one circumstance: my own.

Another approach to the problem of other minds is to deny what I took for granted above: that we do not directly observe other people's psychological states. According to this line of thought, when I watch the baseball player writhing around on the ground, I do not merely infer that he is in pain: I see that he is in pain. One way to cash this out is to adopt some form of "behaviorism": to define psychological states in terms of physical behavior. One worry about this approach is the one you raised in your question: how can I see that he is in pain, if he could look exactly as he does now when in fact he's just faking it? As you say, there seems to be a difference between being in pain and merely acting as if one is in pain.

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