Here be hallucinations from without.  But when a patient begins with a hallucination of the intellect, as that inquisitors are plotting to catch him, or witches to enchant him, and when he later comes to see inquisitors and witches, where there are none, we have, apparently, a hallucination from within.  Again, some persons, like Blake the painter, voluntarily start a hallucination.  ‘Draw me Edward I.,’ a friend would say, Blake would, voluntarily, establish a hallucination of the monarch on a chair, in a good light, and sketch him, if nobody came between his eye and the royal sitter.  Here, then, are examples of hallucinations begotten from within, either voluntarily, by a singular exercise of fancy, or involuntarily, as the suggestion of madness, of cerebral disease, or abnormal cerebral activity.

Again a certain amount of intensity of activity, at a ’sensory centre’ in the brain, will start a ‘percept’.  Activity of the necessary force at the right place, may be normally caused by the organ of sense, say the eye, when fixed on a real object, say a candlestick. (1) Or the necessary activity at the sensory centre may be produced, abnormally, by irritation of the eye, or along the line of nerve from the eye to the ‘sensory centre’. (2) Or thirdly, there may be a morbid, but spontaneous activity in the sensory centre itself. (3) In case one, we have a natural sensation converted into a perception of a real object.  In case two, we have an abnormal origin of a perception of something unreal, a hallucination, begotten from without, that is by a vice in an external organ, the eye.  In case three, we have the origin of an abnormal perception of something unreal, a hallucination, begotten by a vicious activity within, in the sensory centre.  But, while all these three sets of stimuli set the machinery in motion, it is the ‘highest parts of the brain’ that, in response to the stimuli, create the full perception, real or hallucinatory.

But there remains a fourth way of setting the machinery in motion.  The first way, in normal sensation and perception, was the natural action of the organ of sense, stimulated by a material object.  The second way was by the stimulus of a vice in the organ of sense.  The third way was a vicious activity in a sensory centre.  All three stimuli reach the ‘central terminus’ of the brain, and are there created into perceptions, the first real and normal, the second a hallucination from an organ of sense, from without, the third a hallucination from a sensory centre, from within.  The fourth way is illustrated when the machinery is set a-going from the ’central terminus’ itself, ’from the higher parts of the brain, from the seats of ideation and memory’.  Now, as long as these parts only produce and retain ideas or memories in the usual way, we think, or we remember, but we have no hallucination.  But when the activity starting from the central terminus ‘escapes downwards,’ in sufficient force, it reaches the ‘lower centre’ and the organ of sense, and then the idea, or memory, stands visibly before us as a hallucination.