What is green? Imagine I should present to you an object which, to my mind, is of indisputable greenness and ask, "Does this thing appear to you to be green?" Naturally, you might say that it does, for you have come to recognize the appearance of the color of the object to be "green," associating the word with what your eyes see. But could it be my understanding of "green" differs entirely from yours? What if, perchance, you could see into my mind? You might realize that all things that I name "green" are actually "red" in your understanding.