I think that the important point here is that there are huge similarities about how all human beings see things (especially if you count out such edge cases as blind people, the insane, etc.) and that this foundation is something that an artist can build on. If everyone saw things completely differently then an artist would have no way to portray much of anything because there would be no sensory common ground on which to found his creative expression. Hands look a certain way to everyone. This means that if an artist paints a hand in a certain way he can count on this common experience we have to evoke similar mental states, and to evoke similar differences in mental states if he represents it in different ways, either subtle or overt. A fist, a pale sickly hand, an accusatory pointing finger, one withered with age or bursting with youth, are all ways of representing a hand with a million shades of difference between them and with hundreds of techniques for expressing subtle differences between them it is an absolutely essential premise of painting (or any art for that matter) that we are in fact making objectively different changes in the medium in which we are working and that the audience is seeing objectively the same thing that is being produced. Take away that common ground of objectivity and there's no reason to prefer Bouguereau over Barbecue.