Modus ponens

If you read this blog post, you will learn how to turn weak statements into valid arguments.

"Cigarettes are bad." How many of the 1 billion smokers in the world will change their habit after hearing this sentence? Presumably nobody because it requires more than a phrase to kick nicotine dependence. How many teenagers will it convince not to start? Presumably few unless good reasons and compelling evidence back this claim up. Once we have found all three elements of each argument, we need to check whether they are true.

Two types of valid arguments are a modus ponens or a modus tollens. By following the “if a, then c” format, you present premises that affirm a claim. In the case of smoking, this would be “If people smoke, they will die.” You can use the opposition version - a modus tollens - when you want to deny the consequent (If  c is false, a has to be wrong).

How to check the logic of arguments

1. Write down your arguments in an “if a, then c” or “If c is wrong, then a is wrong” format.
(a= antecedent, c= consequent)

2. Check what you are trying to affirm or deny.
(Note that denying the antecedent or affirming the consequent would be invalid arguments.)

3. If you are trying to affirm an antecedent (modus ponens), look for rebuttals if anyone comes up with alternative explanations. If you aim to reject a consequence (modus tollens), make sure that you also prove all premises wrong.

Exercise

What is the right way to legalise weed? Read the NYT opinion piece, identify three arguments (incl. their antecedents and consequents) and classify them (modus ponens/ modus tollens/ invalid).

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