Allan Kardec. The book of spirits (№ 396481)

Question: Why don't we always remember what we dreamed?
Answer: what you call a dream, there is just something that is nothing more than the rest of the body, for the spirit is in constant movement; there he is regaining something of his freedom and deals with those who are dear to him either in this world or in other worlds; but as the body is a derivative of matter heavy and rough, it barely keeps the impressions of the spirit, for the spirit is received, not through the organs of the body.