Also abbreviated MP or MPP, is one of the most elementary (valid) logical inferences. It is based on a conditional and the affirmation of its antecedent.
A → B–if A, then B.
A–A is true.
∴ B–therefore B [is true].
The following is an example of a valid Modus Ponens:
If it is raining, [then] the street will get wet.
It is raining,
Therefore: The street will get wet.
The full name of this form is “Modus ponendo ponens”. Loosely, this could be translated as the “mode of inferring an affirmative [consequent statement] by affirmation [of the antecedent statement]”.
Although the MP is intuitively understandable to most people, it is not uncommon for people to draw erroneous conclusions based on it.
The following table contrasts the modus ponens with its most common fallacies:
| Modus ponens (valid inference) | Affirming the consequent (formal fallacy) | Denying the antecedent (formal fallacy) |
||
|---|---|---|---|---|
| Premise 1 | A → B (if A, then B) | A → B (if A, then B) | A → B (if A, then B) |
|
| Premise 2 | A | B | ⌐A (not A) | |
| Conclusion | B | | |