How do we know all these things? How do you know that when you cut a branch off a tree it doesn't hurt the tree -- only it can't express its pain because it can't move? (Or maybe it loves having its branches pruned.) How do you know that the muscle cells in your heart don't feel pain or excitement when you run up a flight of stairs? How do you know that a kleenex doesn't feel anything when you blow your nose into it?
[What does it all mean? Thomas Nagel]
What does the word "only" mean in the context?
I think it means "but". Is it correct?