Rivers of ink have been spilled over dreams and we still don't know what they mean, if they mean anything. Perhaps they are distorted reflections of reality, or a window into the activity of the subconscious.

Evolutionary reasoning, however, leads us to a safe conclusion. If we share dream activity with dogs, cats and many other animals, it means that dreams have persisted for tens or hundreds of millions of years on this harsh planet. If dreams are hallucinations, what guarantees that our conscious experience is not another hallucination?