What Is Modus Ponens?


Modus ponens is a valid argument form. Because the form is deductive and has two premises and a conclusion, modus pones is an example of a syllogism. (A syllogism is any  deductive argument with two premises and a conclusion.)

The Latin phrase ‘modus ponens‘, translated literally, means ‘mode of affirming’.

Shown schematically, this form of argument looks like this:

Premise 1: If A then B.

Premise 2: A.

Conclusion: Therefore, B.

Arguments of this form are produced by substituting statements in English for A and for B. For example, suppose A = ‘Casey is a dog’ and B = ‘Casey has four legs.’ We can substitute as follows, for a valid argument:

Premise 1: If Casey is a dog, then Casey has four legs.

Premise 2: Casey is a dog.

Conclusion: Therefore, Casey has four legs.

Any argument of this form is valid. But not every argument of this form is sound. For an argument to be sound, it must meet two requirements. First, it must be valid; second, it must have true premises. The above argument about Casey is valid, but it’s not sound. Why? Because the first premise is false. It implies that all dogs have four legs. But this generalization, unfortunately, is not true.

Because modus ponens arguments are always valid, we may extrapolate from this argument form a rule of inference as follows:

“Always infer B from the conjunction of two premises, if one premise is a conditional statement of the form ‘If A, then B,’ and the other premise affirms A.” (The order of the premises doesn’t matter.)

Caveat:

Be careful not to confuse modus ponens with modus tolendo ponens. Modus tolendo ponens is an argument of the following form:

Premise 1: Either A or B.

Premise 2: Not-A.

Therefore, B.

See also “What Is Modus Tollens?”

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Teaching Logic & Critical Thinking to Your Kids


Cover of

Cover of Being Logical: A Guide to Good Thinking

It’s pleasing to know that parents are taking a more proactive role in the education of their children, whether or not they are homeschooling. I’ve been asked if I can recommend tools that could be used to teach children the elements of logic and critical thinking.

  1. My first suggestion is that the best way to teach children how to think critically is to be a visible model of critical thinking. Children have a far greater aptitude for critical thinking than adults credit them for. They tend to be good at inferential reasoning. Their powers are limited in part by their limited storehouse of information from which to make inferences.
  2. Modeling excellence in critical thinking presupposes skill in critical thinking. So parents need to be students of logic and critical thinking themselves. Unfortunately, most have not had the opportunity for formal education in these skills. But there are accessible books to consider. I’ll add a list of recommendations at the end of this post.
  3. If your children see you making the attempt to sharpen your skills in reasoning, this will itself be a good example to them. You can tell them what you’re learning.
  4. Learn the names of basic inferential moves (for example modus ponens, modus tollens) and use these labels with your children when they demonstrate their own ability to make such moves. This should reinforce their awareness of the significance of their mental powers, and affirm them in the use of their powers.
  5. Encourage your children to think about the implications of something they have said or heard. You’ll have to be alert to opportunities for this. But once you’ve been at it for awhile, you’ll get into a natural groove. It will eventually become a part of your routine interaction with your kids. How to do this? I’ll save that for another post sometime.
  6. Get your children reading at their grade level (or above!) books that exemplify and encourage critical thinking. Mystery and suspense novels, carefully selected for their sophistication and interest, can be useful. I read the Hardy Boys as a kid. I also liked the stories of the Sugar Creek Gang.
  7. If you’re home schooling (or not), you can include in the curriculum some materials that teach critical thinking. The Fallacy Detective is a good source for this. (See below.)

Recommendations:

So, here are a few of the many resources available. I’m recommending those that provide a good place to start. Each title is linked to its Amazon page.

Books that inspire parents and other educators to teach children these skills:

Books for self-education in logic and critical thinking:

With adequate preparation in the early years, children in junior high and high school may be ready to work through these books themselves. They don’t provide a complete education in logic, but they are satisfactory for pre-college preparation. For more rigorous study in high school, I recommend using one of two textbooks:

Like most textbooks, Copi and Hurley are pricey. So you may want to settle for a second-hand copy. The illustrations and exposition of old editions will be dated, but the logic will be the same! I shop for second-hand books at AbeBooks.com.

For grade school and up:

Fiction classics for youth:

This post is cross-referenced in an interesting post here.

Related Posts by Doug Geivett:

Clever Edit of Mexico Debate a Challenge to Richard Dawkins


Apparently, Birdie.com has edited the YouTube video made of the debate in Mexico from November 2010, in which I, Bill Craig, and David Wolpe debated Richard Dawkins, Michael Shermer, and Matt Ridley. The aim of this 6 minute feature is to show the mistakes that Richard Dawkins made in understanding and assessing our case in that debate.

If you wish to see this clip, click here.

Interview with Brian Auten


I was interviewed recently by Brian Auten. Most of Brian’s questions concern the topic of miracles. Today Brian has posted this audio interview at his website and can be heard here.

Doug’s other posts on the subject of miracles:

My Right to Park My Car in Front of My House


There’s a debate in my town about whether residents should be free to park their vehicles in the street in front of their homes, without threat of getting a parking ticket.

Last weekend, The Orange County Register published a story about this— Read more of this post

The Apologetics of Jesus


What would Jesus do if he was alive on the earth now and facing the skeptics of our day? The same thing he did in the first century. And what was that?

This question is answered with great clarity in the new book by Norman Geisler and Patrick Zukeran—The Apologetics of Jesus: A Caring Approach to Dealing with Doubters.

I want to recommend this book for several reasons: Read more of this post

Does “Somewhere in Between” Mean “Ideologically Neutral”?


At Politico.com, Michael Calderon has a piece assessing the significance of the drop in viewership at CNN—“CNN fades in prime-time picture.” The brief article is mostly just straight reporting.

  1. Viewers seem to rely on CNN the most at election time, while turning to other cable networks during the long intervals between elections.
  2. CNN just won a Peabody Award.
  3. Doubts have been raised about whether CNN will be able to compete with MSNBC and FOX.
  4. CNN president Jon Klein says yes and that he can explain evidence to the contrary.
  5. Anderson Cooper is CNN’s most valued trick pony, followed by Campbell Brown (who’s about to return from maternity leave).
  6. Cooper’s ratings have fallen off dramatically in recent months, and it’s expected that this will continue.
  7. CNN staffers and former staffers report that concerns within the ranks are greater than reported by Klein.
  8. The critical demographic is viewers ages 25-54.

These are the “facts”—except for the part about the trick ponies, which I slipped in. And there’s a reason why I use the term “trick pony” to refer to cable TV “news” anchors. To begin, the persona of an anchor is crucial to nabbing and keeping viewers. Everyone acknowledges that. But we should wonder why.

The answer may seem obvious. Take CNN, for example. They claim to be “the most trusted news . . .” Leave aside the question whether the tag captures the truth. Why would they be trusted more than the other networks? Remember, the answer has to have something to do with persona. So why would Anderson Cooper, the leading news anchor for CNN, be, in effect, the most trusted news reporter, period?

The answer we’re supposed to come up with is that CNN is ideologically neutral, and Anderson Cooper is the embodiment of that neutrality. And, we must remember, ideological neutrality is good . . . if it’s news you want.

Calderon begins to reveal this outlook early on, when he contrasts the CNN strategy with the “more opinionated programming” at FOX and MSNBC. Notice that—FOX and MSNBC are “more opinionated” in their programming. Maybe that’s true. But what does it mean, and why believe it?

Well, a network can be more or less opinionated. FOX and MSNBC are “more.” So CNN is “less.” Thus, it follows that CNN may also be airing “opinionated programming,” but just not as much as FOX and MSNBC. But then, what is this more or less of opinionated programming? And are viewers supposed to be able to tell when it’s happening and when it isn’t?

Surely things aren’t that simple.

I think we can agree that Keith Olberman is an opinionated guy, and that he unleashes his opinions pretty regularly on his show at MSNBC. Sean Hannity comes to mind when thinking of FOX. So does Bill O’Reilly, who has created a whole new meaning for the phrase “I’ll let you have the last word.” (If you’re a guest with whom he disagrees, he will, indeed, “let you have it.”)

We agree in thinking that prominent anchors at MSNBC and FOX are “opinionated” because it’s obvious. But here’s the significant point: what’s obvious is what their opinion is. That is, they make it obvious that they are presenting an “opinion” because they tell us when they are giving us their opinion.

Why is this so significant? Because opinions don’t always come flying at us with banners telling us that we’re in the trajectory of an opinion. Often they sneak up on us, clothed with disclaimers that their message is completely “objective.”

Calderon is mistaken in suggesting that CNN is ideologically neutral on the grounds presented by him in his piece. Being neither overtly conservative nor overtly liberal, in the style of FOX and MSNBC, respectively, does not mean that CNN is “in the middle” or “neutral.” It has been convincingly argued that they are not neutral but considerably left of center.

Viewers need skills in detecting the ideological commitments of media outlets, the more so when their commitments are more subtly packaged and publicly advertised as “neutral.”

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