In classical logic, disjunctive syllogism (historically known as modus tollendo ponens) is a valid argument form which is a syllogism having a disjunctive statement for one of its premises.
- The breach is a safety violation, or it is not subject to fines.
- The breach is not a safety violation.
- Therefore, it is not subject to fines.
In propositional logic, disjunctive syllogism (also known as disjunction elimination and or elimination, or abbreviated ∨E), is a valid rule of inference. If we are told that at least one of two statements is true; and also told that it is not the former that is true; we can infer that it has to be the latter that is true. If P is true or Q is true and P is false, then Q is true. The reason this is called "disjunctive syllogism" is that, first, it is a syllogism, a three-step argument, and second, it contains a logical disjunction, which simply means an "or" statement. "P or Q" is a disjunction; P and Q are called the statement's disjuncts. The rule makes it possible to eliminate a disjunction from a logical proof. It is the rule that:
where the rule is that whenever instances of "", and "" appear on lines of a proof, "" can be placed on a subsequent line.
Disjunctive syllogism is closely related and similar to hypothetical syllogism, in that it is also type of syllogism, and also the name of a rule of inference. It is also related to the Law of noncontradiction and the Law of excluded middle, two of the three traditional laws of thought.
The disjunctive syllogism rule may be written in sequent notation:
where , and are propositions expressed in some formal system.
Natural language examples
Here is an example:
- I will choose soup or I will choose salad.
- I will not choose soup.
- Therefore, I will choose salad.
Here is another example:
- It is red or it is blue.
- It is not blue.
- Therefore, it is red.
Inclusive and exclusive disjunction
Please observe that the disjunctive syllogism works whether 'or' is considered 'exclusive' or 'inclusive' disjunction. See below for the definitions of these terms.
There are two kinds of logical disjunction:
- inclusive means "and/or" - at least one of them is true, or maybe both.
- exclusive ("xor") means exactly one must be true, but they cannot both be.
The widely used English language concept of or is often ambiguous between these two meanings, but the difference is pivotal in evaluating disjunctive arguments.
- P or Q.
- Not P.
- Therefore, Q.
is valid and indifferent between both meanings. However, only in the exclusive meaning is the following form valid:
- Either (only) P or (only) Q.
- Therefore, not Q.
However, if the fact is true it does not commit the fallacy.
With the inclusive meaning you could draw no conclusion from the first two premises of that argument. See affirming a disjunct.
Related argument forms
Unlike modus ponendo ponens and modus ponendo tollens, with which it should not be confused, disjunctive syllogism is often not made an explicit rule or axiom of logical systems, as the above arguments can be proven with a (slightly devious) combination of reductio ad absurdum and disjunction elimination.
Other forms of syllogism:
- Copi, Irving M.; Cohen, Carl (2005). Introduction to Logic. Prentice Hall. p. 362.
- Hurley, Patrick (1991). A Concise Introduction to Logic 4th edition. Wadsworth Publishing. pp. 320–1.
- Copi and Cohen
- Sanford, David Hawley. 2003. If P, Then Q: Conditionals and the Foundations of Reasoning. London, UK: Routledge: 39
- Copi and Cohen
- Moore and Parker
- Chris Mortensen, Inconsistent Mathematics, Stanford encyclopedia of philosophy, First published Tue Jul 2, 1996; substantive revision Thu Jul 31, 2008