I can see it, hear it, feel it, taste it - but I can never be on the inside of it with you. I cannot even be sure whether I really know what it is like. Is it 'like' my own? Or incomparable? Just as I can never know if what you see at any given moment is exactly the same as what I see. We look at a colour. We both call it red. But it is only because we have been taught to call it by that name. There is no guarantee - not ever - that we see it in the same way, that your red is my red.