In tort law, unreasonable ignorance is a defense based upon the principle that the claimant's actions were uninformed, dangerous, or irrational to a gross degree and could not have been reasonably foreseen by the defendant. The defense most often arises in claims of negligence but also applies to malicious intent. In civil law jurisdictions, unreasonable ignorance is referred to as fatuitas profundis or occasionally culpa asini.^[1]

Unreasonable ignorance holds that a party's unforeseeable inability to act in their own interest cannot lead to damages against a defendant, should that defendant have been inculpable if its actions were applied to a reasonable third-party. Although related to contributory and comparative negligence, unreasonable ignorance does not require partial fault on the part of the claimant; in fact, many relevant situations preclude the claimant from being at fault precisely due to their gross incompetence.

A well known case in which unreasonable ignorance was successfully argued is Groening v. Silicate Products, Inc., 1988. While employed at a facility that produced asbestos-based construction materials, Groening would often place fragments of those materials in his mouth (for unknown reasons).^[2] Groening was an otherwise competent worker and had been provided with safety equipment. After Groening developed esophageal cancer, his former employer was found free of fault due to the unforeseeable irrationality of Groening's actions.^[3]