Duhaime's Law Dictionary

Delusion Definition:

A firm yet irrational belief and which may affect an individual's capacity to contract.

In Forman, Justice Macdonald adopted these words:

"Where the patient conceives something extravagant to exist which has still no existence, whatever, but in his own heated imagination, and where at the same time having once so conceived he is incapable of being or at least of being permanently reasoned out of that conception, such a patient is said to be under a delusion."

In Re Watts, New Brunswick Judge Grimmer referred to Banks v Goodfellow (cited below) and summarized the law as follows:

"... a delusion has been defined as a false belief originating spontaneously in the imagination without foundation either in fact or evidence, of the falsity of which the person affected cannot be convinced by argument or proof.

"Also there is abundant authority for the statement that an insane delusion is a belief which has no basis in reason and which cannot be dispelled by argument.

"There is also ample authority for and it has been laid down that a mistaken belief as to a matter of fact or illogical conclusions therefrom is not necessarily an insane delusion, neither is any belief or prejudice however mistaken which has some basis for it.

"A person may have a delusion which does not imply or show unsoundness of mind, and even mistaken unnatural prejudice may not be an insane delusion, all persons being subject to likes and dislikes.

"The mere existence, however, of an insane delusion will not in validate a will unless shown to affect its provisions...."

More recently, the Attorney's Dictionary of Medicine defines the term as:

"A wrong belief, contrary to facts, which cannot be rectified by reasoning, persuasion or evidence acceptable to the patient, or even by the patient's own observations."

The existence of delusion in a person will often be a harbinger of more pervasive mental illness and incapacity. However, on the basis of a delusion alone, mental capacity is not necessarily vacant.

In Re Bohrmann, the English court noted that:

"... the law has recognised ... that a man may make a perfectly good will so long as the delusions from which he suffers have no relation to any testamentary capacity."

In this area of the law, the line is fine indeed and reflects the court's ongoing desire to do whatever they can to avoid voiding wills, small delusion notwithstanding.

In Banks v Goodfellow, the Court noted that the testator was convinced of the actions of evil spirits yet the will was upheld by the court because of a lack of nexus between the delusion and the disposition of property in the contested will.

But in Smee v Smee, a testator was deluded in thinking a stranger was his brother and left his estate to "his brother". The will was set aside.


  • In re The Estate of Bohrmann [1938] 1 All ER 271
  • Banks v Goodfellow 1870 QB 549 (England)
  • Forman v Ryan [1912] 4 DLR 27 (BCCA)
  • Schmidt, J., Attorney's Dictionary of Medicine (New Jersey: LexisNexis, 2001).
  • Smee v Smee (1879) 5 PD 84 (England)

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