The dreamer generally dreams of things which have lately been weighing on his mind, but not necessarily so, nor does it follow that he will dream what has been ardently expected or painfully dreaded. Association of ideas may lead his unguided mind to a scene or object which, in his wakeful moments, he cannot trace, for his memory usually preserves only the final objects or scene, and not the various steps that led to it. Thus, if moving be on his mind, he may, in his dream, see a moving van, then a painting on the side of the van, then an artist, then a paint shop, a model, another picture on an easel, and finally a very pleasant or a very horrible scene in a studio. When the dreamer awakes he remembers only the scene, and he is at a loss to know why he should have dreamed of a scene so foreign to his previous thoughts. There appears to be no truth whatever in the theory that dreams come as omens or warnings, for they are purely accidental. Neither is there apparently any truth in the belief that dreams come by opposites, that they are the manifestation of some invisible agency, or that there is anything supernatural, uncanny or mysterious about them. To maintain that one can foretell future events, or read past events, from dreams, is absurd. Nearly every person dreams each night, and particularly during the moments when losing consciousness and the moments when awakening, since imperfect sleep then obtains; and, it would be strange indeed if, during one or more of these occasions, we did not by chance dream of something which afterwards actually happens. All bodily derangements that interrupt healthy sleep, such as irritation of the digestive organs, and even over-exertion, worry, and undue excitement, will produce dreams, and it is therefore fairly obvious that, since we know the cause of dreams, their effects and results, there is nothing marvellous, unnatural, wonderful, extraordinary or supernatural in dreams. Until the past few hundred years, the cause of dreams was not understood. Aristotle believes the cause of dreams to be common sense, but placed in the fancy. Avicen thought it to be an ultimate intelligence moving the moon in the midst of that light with which the fancies of men are illuminated while they sleep. Averroes, an Arabian physician, ascribed it to the imagination. Democritus referred the cause of them to little images, or representations, separated from the things themselves. Plato placed it among the specific and concrete notions of the soul. Albertus attributed dreams to superior influences, which continually flow from the sky, through many specific channels.