40. Theory of hallucination. Another opinion, less offensive, inasmuch as it
bears on its surface a colour of scientific discrimination, attributes these phenomena to
illusion of the senses. Those who hold it say: "The observer may be a very respectable
person; but he thinks he sees what he does not see. When he sees a table rise up and
remain in the air, without anything to rest upon, the table does not really move at all; he
sees it in the air by a sort of mirage, or by some effect of refraction, like that by which
we see a star, or an object in the water, Out of its true position." Such an illusion would
be possible in point of fact, but witnesses of these phenomena are able to prove their
objective nature, by passing under the suspended table, which would be difficult, if it
had not quitted the floor. On the other hand, it often happens that the table is broken in its fall to the floor; can such a breakage be the effect of an optical illusion?
A well-known physiological cause may undoubtedly make us believe that we
see a thing turn which does not move; or a man attacked with vertigo may fancy
himself to turn when he is stationary; but when several persons are witnesses to the same fact, can it be alleged that all such persons are the victims of illusion ?