Meaning: the act of causing something to happen
When habit and past experience play this part, we are in the region of mnemic as opposed to ordinary physical causation.
These definitions would have all the precision that could be desired if the distinction between physical and psychological causation were clear and sharp.
And I think that, if we could regard as ultimately valid the difference between physical and mnemic causation, we could distinguish images from sensations as having mnemic causes, though they may also have physical causes.
In these children's dreams nothing points to the influence of such somatic stimuli; we cannot be mistaken, for the dreams are entirely intelligible and easy to survey. But we need not give up the theory of physical causation entirely on this account.
However this may be, the practically effective distinction between sensations and images is that in the causation of sensations, but not of images, the stimulation of nerves carrying an effect into the brain, usually from the surface of the body, plays an essential part.