How to Recognize the Modus Ponens Argument Form

An argument form is simply a pattern of reasoning. To think critically, you must be able to recognize an argument form. There are several well-established forms of argument; one of the most common is the modus ponens form of argument. Being able to recognize and analyze this type of argument is important to critical analysis and thinking.

Difficulty:

Moderate

Instructions

1. Consider the following two arguments:

   Argument #1:
   1. If Lassie is a collie, then Lassie is a dog.
   2. Lassie is a collie.
   Therefore, 3. Lassie is a dog.

   Argument #2:
   1. If Barack Obama is a U.S. president, then Barack Obama is a U.S. citizen.
   2. Barack Obama is a U.S. president.
   Therefore, 3. Barack Obama is a U.S. citizen.

2. In each of the example arguments, lines 1 and 2 are the premises and line 3 is the conclusion. Both of these arguments are valid, and it is necessary that, if the premises are true, the conclusion is true. Both of these arguments also have the same form. This particular form of reasoning is so commonly used that logicians have given it a special name: modus ponens, which means "the way or mode of positing."

3. Notice that in each of the arguments, the second premise posits or affirms the "if" part of the first premise. This pattern of reasoning can be represented as follows:

   1. If A, then B.
   2. A.
   Therefore, 3. B.

   Here, the letters A and B are variables that stand in for statements. Plug in any statement for the "equation," and it will still remain true.

Tips & Warnings

• All of these arguments show the fact that the validity of an argument that has the form of modus ponens is guaranteed by that form alone. Its validity does not depend on its subject matter or content.

References

• Changing Minds: Modus Ponens
• Wolfram MathWorld: Modus Ponens
• PlanetMath: Modus Ponens