122 Inquiries into Human Faculty
and motionless, just as though it had always been there, at a spot on which my eye had rested a moment previously and seen nothing. Again, after a while my eye wandered, and on its returning to the spot the seal was gone. The water had closed in silence over its head without leaving a ripple, and the sheen on the surface of the pond was as unbroken as when I first reached it. Where did the seal come from, and whither did it go? This could easily have been answered if the glare had not obstructed the view of the movements of the animal under water. As it was, a solitary link in a continuous chain of actions stood isolated from all the rest. So it is with the visions; a single stage in a series of mental processes- emerges into the domain of consciousness. All that precedes and follows lies outside of it, and its character can only be inferred. We see in a general way that a condition of the presentation of visions lies in the over-sensitiveness of certain tracks or domains of brain action and the under-sensitiveness of others, certain stages in a mental process being represented very vividly in consciousness while the other stages are unfelt; also that individualism is changed to dividualism.
I do not recollect seeing it remarked that the ordinary phenomena of dreaming seem to show that partial sensitiveness is a normal condition during sleep. They do so because one of the most marked characteristics of the dreamer is the absence of common sense. He accepts wildlyincongruous visions without the slightest scepticism. Now common sense consists in the comprehension of a large number of related circumstances, and implies the simultaneous working of many parts of the brain. On the other hand, the brain is known to be imperfectly supplied with blood during sleep, and cannot therefore be at full work. It is probable enough, from hydraulic analogies, that imperfect irrigation would lead to partial irrigation, and therefore to suppression of action in some parts of the brain, and that this is really the case seems to be proved by the absence of common sense during dreams.
A convenient distinction is made between hallucinations and illusions. Hallucinations are defined as appearances wholly due to fancy; - illusions, as fanciful perceptions of
