The Modus Ponens Argument

The Modus Ponens argumentative form is so common to us as thinkers that it is oftentimes easy to overlook it’s vital importance to our every day reasoning. This argumentative form in it’s original Latin means to “affirm by affirming.” Simply put, this means that we can affirm some kind of conclusion (announce that it is true) by affirming something else. Today let’s look at this structure and one of it’s associated fallacies.

The Modus Ponens structure looks something like this:

  1. If P–>Q
  2. P
  3. Therefore, Q

Premise 1) is what is known as a conditional premise. A conditional premise has two parts: an antecedent and a consequent. Premise 2) is a simple truth assertion. In this case, premise 2) validates the antecedent of the conditional, leading us to conclude that Q is necessarily true. If we affirm the antecedent P, we then affirm Q. Assuming that premise 1) and 2) are really true in real life, let’s look at how this logic plays out:

  1. If you are reading this blog post, then you are on the computer.
  2. You are reading this blog post.
  3. Therefore, you must be on the computer.

A common misapplication of this structure is what is known as affirming the consequent. Let’s examine a similar structure below:

  1. If P–>Q
  2. Q
  3. Therefore, P

Notice the difference in premise 2). Premise 2) affirms the consequent, not the antecedent.  Using the same real life scenarios as above, let’s insert them into this structure and we will see that it is indeed a fallacy.

  1. If you are reading this blog post, then you are on the computer.
  2. You are on the computer.
  3. Therefore, you must be reading this blog post.

Notice that in this case, all we know is that you are on the computer-we don’t know that you are reading this blog specifically. You could be checking the weather, buying plane tickets, or listening to Rick Astley. We simply can’t know what you are doing on the computer. All we know is that you are on it. In this case, it is easy to see that it does not logically follow that you must be reading this blog post, although it is possible. This kind of conclusion is a non-sequitur, which means that the conclusion “does not follow” from the premises.

 

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