These are, no doubt, the conditions of a very large number of our dream-images.

There must, of course, be some primary cerebral excitation, whether that of a present peripheral stimulation, or that which has been termed central and spontaneous; but when once this first link of the imaginative chain is supplied, other links may be added in large numbers through the operation of the forces of association.

One may, indeed, safely say that the large proportion of the contents of every dream arise in this way.
The very simplest type of dream excited by a present sensation contains these elements.

To take an example, I once dreamt, as a consequence of the loud barking of a dog, that a dog approached me when lying down, and began to lick my face.

Here the play of the associative forces was apparent: a mere sensation of sound called up the appropriate visual image, this again the representation of a characteristic action, and so on.