It’s kind of like the sight thing. I call the sky blue. I have been taught since I was a child that the sky and things that are the same color are called “blue”. We’ve all been taught that. So you call the sky and similarly colored things blue as well. But maybe you’re really seeing what I know as yellow. Maybe our eyes interpret the colors differently but we’ve all been shown the same objects and taught their colors. But our colors could all be different.
But there is no way to know. You can only know what you’re seeing and what you’re feeling. And to compare wouldn’t work. Because we all define happiness based on how we’ve been taught happiness feels. So when you were an infant and someone gave you a cookie and said “Now aren’t you happy?”, you learned that whatever you were feeling then was happiness. And that it feels good. But that could be completely different from what I was feeling.
And this could go on forever. We can doubt everything we know and feel. But there’s nothing we can do about it.