In either case, what causes the hallucination, or are there various possible sorts of causes? Now defects in the eye, or in the optic nerve, to speak roughly, may cause hallucinations _from without_. An injured external organ conveys a false and distorted message to the brain and to the intelligence. A nascent malady of the ear may produce buzzings, and these may develop into hallucinatory voices. 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.