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Deciding Between Arguments


Surely you’ve heard the saying, “One man’s trash is another man’s treasure.”


But, I bet most of you haven’t heard the dorky philosopher’s version: “One man’s modus ponens is another man’s modus tollens.”


“Clint, please, it’s too early for this.”


Bear with me! You might find this helpful.


Modus ponens is a classic, valid argument structure. It goes like this:


Premise 1: If A, then B.

Premise 2: A.

Conclusion: Therefore, B.


Substitute ANYTHING in for the variables A and B, and you will have a logically valid argument. Of course, the argument may not be sound, for that we’d need true premises and valid structure. But, at the very least, modus ponens is a valid argument structure.


“Clint…please stop...”


I know, I know, but check out it’s close cousin, modus tollens:


Premise 1: If A, then B.

Premise 2: not-B.

Conclusion: Therefore, not-A.


Again, pick anything to fill in the variables, and you’d still have a valid argument structure. Pretty cool, right?


“Clint, get to the point - you have 10 seconds.”


Ok, this can help us think about free will and other big ideas in philosophy and our worldview.


“Fine - I’m listening.”


Consider two arguments - the first is modus ponens and the second is modus tollens. See if you notice anything weird about them.


  1. If determinism is true, we don’t have free will.

  2. Determinism is true.

  3. Therefore, we don’t have free will.

  1. If determinism is true, we don’t have free will.

  2. We have free will.

  3. Therefore, determinism is false.

Notice that the conclusion of the first argument is just the rejection of premise 2 in the second. And, both utilize the exact same “If A, then B” conditional as the first premise.

You might think, “So, which argument is the *right* one?”

Good question. Philosophers have wrestled with how to answer that question. One thought is to spring for the one for whose premises you have the most evidence for. The one you find most compelling. Depending on which “Premise 2” you find more convincing based on your honest assessment of your available evidence is likely the one you will opt for.

This explains the adage, “One man’s modus ponens is another man’s modus tollens.”

I do think that having this idea in your tool belt is helpful in getting closer to intellectual virtue. That’s a big goal for us at Open to Truth. It allows you to recognize the similarities you may share with someone you disagree with - in this case, the shared endorsement of premise 1, which describes the logical link between the truth or falsity of both determinism and free will’s existence.

And, of course, better understanding the reasoning process or how someone you disagree with arrived at their position is going to help tremendously in further conversation and your own intellectual thoroughness.

If you actually want to dive into some arguments about whether those pesky premise 2s are true, you can listen to this week’s episode “Do We Have Free Will?” on Youtube or listen on your favorite podcatcher.

Stay Curious!