According to Lyttle (1986):

"A false belief which is out of keeping with the patient's background and which is unamenable to logic."

Delusions are held with an overwhelming sense of conviction and cannot be corrected by an appeal to reason. They do not occur in neurosis, but are characteristic of psychotic disorders. some types include:

Each of these can be primary or secondary, and systematized or unsystematised.

A bluish-red man sitting in a room with no windows announces that the moon is a full moon tonight. And as it happens, there is indeed a full moon tonight, but the bluish-red man has not ventured out of this room tonight, nor last night, nor in longer than he can remember. He has consulted no sources of information, and surely has not been out to see the moon, and so his recollection of the phase of the moon is not based on seeing the moon at all.

If the bluish-red man made his claim because an intervening ghost or a goblin or god appeared to him and told him it was so, then he'd probably properly be classed as delusional as to the proposition that the moon was full, his coincidentally correct claim originating in a source unhinged from reality (though were he to become aware of that coincidence, it would serve only to reinforce the original delusion).

But suppose the bluish-red man begged his claim because he heard the howl of a wolf, and is aware that wolves howl at the full moon? No delusion could rest upon that pillar so long as he did, in fact hear a howl. He might have mistaken a dog's howl (or a man's) for a wolf's, and he might have mistaken a howl brought on by nothing in particular for one inspired by the fullness of the moon. But unless there was really never any howl at all, his assertion that a howl truly heard signifies a full moon is merely a mistaken connection, not rightly called a delusion.

De*lu"sion (?) n. [L. delusio, fr. deludere. See Delude.]

1.

The act of deluding; deception; a misleading of the mind.

Pope.

2.

The state of being deluded or misled.

3.

That which is falsely or delusively believed or propagated; false belief; error in belief.

And fondly mourned the dear delusion gone. Prior.

Syn. -- Delusion, Illusion. These words both imply some deception practiced upon the mind. Delusion is deception from want of knowledge; illusion is deception from morbid imagination. An illusion is a false show, a mere cheat on the fancy or senses. It is, in other words, some idea or image presented to the bodily or mental vision which does not exist in reality. A delusion is a false judgment, usually affecting the real concerns of life. Or, in other words, it is an erroneous view of something which exists indeed, but has by no means the qualities or attributes ascribed to it. Thus we speak of the illusions of fancy, the illusions of hope, illusive prospects, illusive appearances, etc. In like manner, we speak of the delusions of stockjobbing, the delusions of honorable men, delusive appearances in trade, of being deluded by a seeming excellence.

"A fanatic, either religious or political, is the subject of strong delusions; while the term illusion is applied solely to the visions of an uncontrolled imagination, the chimerical ideas of one blinded by hope, passion, or credulity, or lastly, to spectral and other ocular deceptions, to which the word delusion is never applied." Whately.

 

© Webster 1913.

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