Kyo

That isn't the point. The existence of 'errors' in perception cannot be detected if the error is consistent/universal. For instance, assume there is an absolute standard for color. There is an 'absolute red', 'absolute green', 'absolute blue', that are exactly red, green, and blue, no matter what anyone seems them as. We place an object that is 'absolute red' in front of two people, one who has 'normal' vision, and one who does not. The first person obviously sees the absolute red and believes the object is red. The second person, however, sees 'absolute green' but says the object is red! Why? Because when he was learning what colors were, 'absolute red' objects were placed in front of him and called red. He saw 'absolute green', and he was taught that the color was called red. Therefore, he will always call 'absolute green', red.

It is impossible to determine exactly what he is seeing - and impossible for him to describe it to you in terms of an absolute standard. So long as he does not call two different absolute colors the same color, there is no way to tell that he is actually seeing something different.

Of course, I believe that given a careful dissection/chemical analysis of his eye and brain, we could determine what he is seeing.

So, instead of saying that no two people see the same thing, perhaps the correct statement is that it is impossible to tell what other people see.