Disjunctive syllogism
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A disjunctive syllogism, also known as modus tollendo ponens (literally: mode which, by taking away, affirms) is a valid, simple argument form:
- P or Q
- Not P
- Therefore, Q
In logical operator notation:
- ¬
where 
represents the logical assertion.
Roughly speaking, we are told that at least one of two statements is true; then we are told that it is not the former that is true; so we infer that it has to be the latter that 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 disjunction, which means simply an "or" statement. "Either P or Q" is a disjunction; P and Q are called the statement's disjuncts.
Note that the disjunctive syllogism works whether 'or' is considered 'exclusive' or 'inclusive' disjunction. See below for the definitions of these terms.
Here is an example:
- Either I will choose soup or I will choose salad.
- I will not choose soup.
- Therefore, I will choose salad.
Here is another example:
- Either the Browns win or the Bengals win.
- The Browns do not win.
- Therefore, the Bengals win.
[edit] Inclusive versus exclusive disjunction
There are two kinds of logical disjunction:
- inclusive means "and/or" where at least one term must be true or they can both be true.
- exclusive ("xor") means one must be true and the other must be false. Both terms cannot be true and both cannot be false.
The popular English language concept of or is often ambiguous between these two meanings, but the difference is pivotal in evaluating disjunctive arguments.
This argument:
- Either 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 P or Q (exclusive).
- P.
- Therefore, not Q.
With the inclusive meaning you could draw no conclusion from the first two premises of that argument. See affirming a disjunct.
[edit] Related argument forms
Unlike modus ponendo ponens and modus tollendo tollens, with which it should not be confused, modus tollendo ponens 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.
Modus tollendo ponens should also not be confused with modus ponendo tollens.
Other forms of syllogism:
