The existential fallacy, or existential instantiation, is a

formal fallacy In logic and philosophical logic, philosophy, a formal fallacy is a pattern of reasoning rendered validity (logic), invalid by a flaw in its logical structure. propositional calculus, Propositional logic, for example, is concerned with the meaning ...

. In the existential fallacy, one presupposes that a class has members when one is not supposed to do so; i.e., when one should not assume existential import. Not to be confused with the '

Affirming the consequent In propositional logic, affirming the consequent (also known as converse error, fallacy of the converse, or confusion of necessity and sufficiency) is a formal fallacy (or an invalid form of argument) that is committed when, in the context of a ...

', as in "If A, then B. B. Therefore A". One example would be: "''Every unicorn has a horn on its forehead''". It does not imply that there are any unicorns at all in the world, and thus it cannot be assumed that, if the statement were true, somewhere there is a unicorn in the world (with a horn on its forehead). The statement, if assumed true, implies only that if there were any unicorns, each would definitely have a horn on its forehead.

Overview

An existential fallacy is committed in a medieval

categorical syllogism A syllogism (, ''syllogismos'', 'conclusion, inference') is a kind of logical argument that applies deductive reasoning to arrive at a conclusion based on two propositions that are asserted or assumed to be true. In its earliest form (define ...

because it has two universal premises and a particular conclusion with no assumption that at least one member of the class exists, an assumption which is not established by the premises. In modern logic, the presupposition that a class has members is seen as unacceptable. In 1905,

Bertrand Russell Bertrand Arthur William Russell, 3rd Earl Russell, (18 May 1872 – 2 February 1970) was a British philosopher, logician, mathematician, and public intellectual. He had influence on mathematics, logic, set theory, and various areas of analytic ...

wrote an essay entitled "The Existential Import of Proposition", in which he called this Boolean approach " Peano's interpretation". The fallacy does not occur in

enthymeme An enthymeme (, ''enthýmēma'') is an argument with a hidden premise. Enthymemes are usually developed from premises that accord with the audience's view of the world and what is taken to be common sense. However, where the general premise of a s ...

s, where hidden premises required to make the

syllogism A syllogism (, ''syllogismos'', 'conclusion, inference') is a kind of logical argument that applies deductive reasoning to arrive at a conclusion based on two propositions that are asserted or assumed to be true. In its earliest form (defin ...

valid assume the existence of at least one member of the class.

Examples

* All trespassers will be prosecuted. * Therefore, some of those prosecuted will have trespassed. This is a fallacy because the first statement does not require the existence of any actual trespassers (stating only what would happen if some do exist), and therefore does not prove the existence of any. Note that this is a fallacy whether or not anyone has trespassed.

References

External links

Fallacy files: existential fallacyFOLDOC: existential fallacy
{{Formal Fallacy Syllogistic fallacies Quantificational fallacies

Overview

Examples

See also

References

External links