Let us say that when you awake tomorrow, you find standing at your bedside a man with purple scale-skin who tells you that he has just arrived from Mars, that he is studying the human species, and that he has selected your mind for the kind of on-the-spot examination he wants to make. While you are catching your breath he walks casually to your best chair, drapes his tail over it, and informs you that he will be visible and audible only to you. Fixing his three eyes sternly upon you, he warns you not to reveal his presence; if you attempt to do so, he threatens, he will kill you instantly.
You may wonder, perhaps, if you are sane. But the Man From Mars is standing before you, clear and colorful, and his voice is loud and distinct. On the basis of what you can so clearly see and hear, you accept the fact, astounding as it is, that the stranger is what he says he is.