Dreams are very different from waking life, but it is extremely difficult clearly to define in what the difference consists. When we are dreaming, we are nearly always convinced that we are awake, and in some cases real experiences have been mistaken for dreams. The latter mistake forms the subject of a celebrated Spanish play called Life a Dream, and of an amusing story in the Arabian Nights, in which a poor man is for a jest treated as a mighty monarch, and it is contrived that he should afterwards think that all the honourable treatment he had actually received was merely a vivid dream. Sometimes even after waking, we may be doubtful whether our dream was a reality or not, especially if we happen to fall asleep in our chair and do not remember the circumstance of having fallen to sleep. Of course this doubt can only arise when there has been nothing in our dream that seems impossible to our wakened mind. It is, however, only in rare cases that a dream exactly copies the experience of our waking hours. As a rule, in our sleep all kinds of events seem to happen which in our waking hours we should know to be impossible. In our dreams we see and converse with friends who are at the other side of the world or have been long dead. We may even meet historical or fictitious characters that we have read about in books. We often lose our identity and dreams that we are someone else, and in the course of a single dream may be in turn several different persons. Space and time to the dreamer lose their reality. It is possible in a dream that lasts a few seconds to appear to have gone through the experience of many years. The limitations of space may also vanish into nothing, so that we seem to travel the most distant parts of the universe with the rapidity of thought. Our imagination gains in some cases such complete control over our reason that we can contemplate all such