Modus Ponens
Modus Ponens is a mode of reasoning from a hypothetical proposition according to which if the antecedent is affirmed, the consequent is affirmed. It means that if one thing is true, then another will be. It can be represented in the following form.
Valid Argument
- If A, then B
- A is true,
- Then B is True
In common language, an example of such an argument is,
- If John is murdered, John is dead
- John is murdered.
- So, John is dead.
However, it is important to note that If A implies B, then the reverse is not necessarily implied. It means that affirming the consequent does not make the antecedent valid.
Invalid Argument
- If A, then B
- B is true,
- Then A is True
In common language,
- If John is murdered, John is dead
- John is dead
- So, John is murdered
Unless you are careful, you may tend to agree with the conclusion of the above argument, though it is invalid. Hence, it is an excellent example of a logical fallacy. However, to establish the above argument as a fallacy, we need one example to establish that either the truth or logic conditions are invalid. This is not a valid argument since John may not have been murdered but could have died of any other reason, like illness, accident, etc. Let’s take another argument.
Argument
- P: If a person is allergic to peanuts, he does not eat peanut butter.
- P: John does not eat peanut butter.
- C: John is allergic to peanuts.
In this case, the conclusion can be wrong even if the premises are correct. For instance, John may not eat peanut butter because he does not like the taste of butter. Hence, just because John does not eat peanut butter does not mean that John is allergic to peanuts. Therefore, the argument is invalid. However, some people won’t understand the error in logic and may get convinced by the argument.
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