You will never know how tall my friend, Nathan is. You will never know how my girlfriend smells after she showers. The fact that I know these things, and you haven't the faintest idea of them, is proof that I exist.
*sigh*
Or nah. My brain hasn't projected these things. They may not even exist but only to you, and you may not even exist. The fact is, we don't know everything even in a powerful state such as the dream state. Senses seem to work in dreams, then you go to recall on those things sensed in a dream and you can't remember it all.