What is modus ponens?

Modus ponens (Latin for ‘mode that affirms’) is a valid form of logical deduction. It is a conditional form (if-then) that we use frequently in everyday reasoning.

We have an antecedent (p) or condition, and we have a consequent (q) or result. If the antecedent is true, then we assert that the consequent logically follows. It takes the form:

Premise 1:       If p the q

Premise 2:       p

Conclusion:     Therefore q

Note that we are affirming p.

For example,

          If it rains then the pavement will be wet.

          It is raining.

          Therefore, the pavement is wet.

This is formally valid; that is, if the premises are true then the conclusion necessarily follows.

If the premises are indeed true, for example we have strong empirical evidence in their favour, then the argument is formally sound.

The following MP is valid but unsound:

          If the Moon is made of cheese then we can make pizza with it.

          The Moon is made of cheese.

          Therefore, we can make pizza with it.

---