> Wait. You mean people thought they didn't?

If you rip the leg off an ant it keeps going as if nothing happened. Or if you cut an earthworm in half it behaves normally. So clearly not all animals feel pain.

The question becomes which ones do, and which don't. You can't ask them, so at best you watch their behavior and look for changes.

The trouble comes in when you try to distinguish between pain and suffering. Pain prompts an animal to change behavior. A worm being shocked will move away - but does that mean it suffers? Or just that it avoids the negative stimulus?

I can't give you the answer, but at least I can help you understand the question.