But reflection could not recognise the unreality
of this 'percept,' till it was found that, in fact, the visitor had
vanished, and had never been in the neighbourhood at all.

Here then, are two classes of hallucinations, those which reflection
shows us to be false (as if a sane man were to have the
hallucination of a crocodile, or of a dead friend, entering the
room), and those which reflection does not, at the moment, show to
be false, as if a friend were to enter, who could be proved to have
been absent.

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