Hypothetical syllogism

In classical logic, hypothetical syllogism is a valid argument form which is a syllogism having a conditional statement for one or both of its premises.
An example in English:
The term originated with Theophrastus.

Propositional logic

In propositional logic, hypothetical syllogism is the name of a valid rule of inference. Hypothetical syllogism is one of the rules in classical logic that is not always accepted in certain systems of non-classical logic. The rule may be stated:
where the rule is that whenever instances of "", and "" appear on lines of a proof, "" can be placed on a subsequent line.
Hypothetical syllogism is closely related and similar to disjunctive syllogism, in that it is also type of syllogism, and also the name of a rule of inference.

Formal notation

The hypothetical syllogism inference rule may be written in sequent notation, which amounts to a specialization of the cut rule:
where is a metalogical symbol and meaning that is a syntactic consequence of in some logical system;
and expressed as a truth-functional tautology or theorem of propositional logic:
where,, and are propositions expressed in some formal system.


Alternative forms

An alternative form of hypothetical syllogism, more useful for classical propositional calculus systems with implication and negation, is the following:
Yet another form is:


An example of the proofs of these theorems in such systems is given below. We use two of the three axioms used in one of the popular systems described by Jan Łukasiewicz.
The proofs relies on two out of the three axioms of this system:
The proof of the is as follows:
The proof of the is given here.

As a metatheorem

Whenever we have two theorems of the form and , we can prove by the following steps: