1)
I think maybe we’ve focused too much on the “happiness” example.
Still, I’ll say this. Eftach, you can think about what you believe makes a person happy, and if you think you feel as good as you should in the situation you’re in (although the way you look at yourself and your situation makes a big difference in happiness, so take that into account). See also Yytz’s post.
2)
I also heard someone ask once that if something is there and no-one is there to see it, is it still there?
The answer science (and possibly Kabbalah) gives for that question might surprise you.
3)
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.
Certainly an interesting concept, but I came up with a way to determine if this is occurring on a large scale.
Certain colors are known to have certain psychological effects – if we assume that our brains’ emotional responses to “objective” colors are uniform, then if enough of us were not seeing the same colors the same way, this could not have been an observable phenomenon. Rather, it would have been observed that not all people have the same emotional responses to color (though it is likely that such a phenomenon still exists between cultures that use various colors in different ways).
The concept is probably still useful in philosophical thought, though.
4)
Does anyone know the name of the concept that we never physically experience something, only our sensual perception of that thing?
(Unless perhaps during an out-of-body/near-death experience…)