When I say that they are illusions, I must define the meaning of the word. An illusion is not the same as an error, it is indeed not necessarily an error. Aristotle’s belief that vermin are evolved out of dung, to which ignorant people still cling, was an error; so was the belief of a former generation of doctors that _tabes dorsalis_ was the result of sexual excess. It would be improper to call these errors illusions. On the other hand, it was an illusion on the part of Columbus that he had discovered a new sea-route to India. The part played by his wish in this error is very clear. One may describe as an illusion the statement of certain nationalists that the Indo-Germanic race is the only one capable of culture, or the belief, which only psycho-analysis destroyed, that the child is a being without sexuality. It is characteristic of the illusion that it is derived from men’s wishes; in this respect it approaches the psychiatric delusion, but it is to be distinguished from this, quite apart from the more complicated structure of the latter. In the delusion we emphasize as essential the conflict with reality; the illusion need not be necessarily false, that is to say, unrealizable or incompatible with reality. For instance, a poor girl may have an illusion that a prince will come and fetch her home. It is possible; some such cases have occurred. That the Messiah will come and found a golden age is much less probable; according to one’s personal attitude one will classify this belief as an illusion or as analogous to a delusion. Examples of illusions that have come true are not easy to discover, but the illusion of the alchemists that all metals can be turned into gold may prove to be one. The desire to have lots of gold, as much gold as possible, has been considerably damped by our modern insight into the nature of wealth, yet chemistry no longer considers a transmutation of metals into gold as impossible. Thus we call a belief an illusion when wish-fulfilment is a prominent factor in its motivation, while disregarding its relations to reality, just as the illusion itself does.