All other previous attempts to solve the problems of dreams have concerned themselves directly with the manifest dream-content as it is retained in the memory. They have sought to obtain an interpretation of the dream from this content, or, if they dispensed with an interpretation, to base their conclusions concerning the dream on the evidence provided by this content. We, however, are confronted by a different set of data; for us a new psychic material interposes itself between the dream-content and the results of our investigations: the latent dream-content, or dream-thoughts, which are obtained only by our method. We develop the solution of the dream from this latent content, and not from the manifest dream-content. We are thus confronted with a new problem, an entirely novel task -- that of examining and tracing the relations between the latent dream-thoughts and the manifest dream-content, and the processes by which the latter has grown out of the former.
The dream-thoughts and the dream-content present themselves as two descriptions of the same content in two different languages; or, to put it more clearly, the dream-content appears to us as a translation of the dream-thoughts into another mode of expression, whose symbols and laws of composition we must learn by comparing the origin with the translation. The dream-thoughts we can understand without further trouble the moment we have ascertained them. The dream-content is, as it were, presented in hieroglyphics, whose symbols must be translated, one by one, into the language of the dream-thoughts. It would of course be incorrect to attempt to read these symbols in accordance with their values as pictures, instead of in accordance with their meaning as symbols. For instance, I have before me a picture-puzzle (rebus) -- a house, upon whose roof there is a boat; then a single letter; then a running figure, whose head has been omitted, and so on. As a critic I might be tempted to judge this composition and its elements to be nonsensical. A boat is out of place on the roof of a house, and a headless man cannot run; the man, too, is larger than the house, and if the whole thing is meant to represent a landscape the single letters of the alphabet have no right in it, since they do not occur in nature. A correct judgment of the picture-puzzle is possible only if I make no such objections to the whole and its parts, and if, on the contrary, I take the trouble to replace each image by a syllable or word which it may represent by virtue of some allusion or relation. The words thus put together are no longer meaningless, but might constitute the most beautiful and pregnant aphorism. Now a dream is such a picture-puzzle, and our predecessors in the art of dream-interpretation have made the mistake of judging the rebus as an artistic composition. As such, of course, it appears nonsensical and worthless.
Freud: The Interpretation of Dreams, Chapter 6