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Definition of "Logical Force"

Leslie Wallace
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Would you pls define the term "logical force" as it is used in the LR Bible? I'm looking at page 516 under "The Validity of the Argument" and wondering if "logical force" equates with valid reasoning. If an argument lacks "logical force," does that mean it is invalid? If an argument exhibits "logical force," does that mean it is a valid argument?

Jonathan Evans
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Hi, Leslie!

Welcome to our forums! Good question!

This discussion is in reference to Parallel the Reasoning questions, but this principle applies across the LSAT.

You are correct, the "logical force" of an argument relates to its validity. The concept of "logical validity" is rather strict. For an argument to be logically valid, the truth of the premises must necessarily entail the truth of the conclusion. As you have likely seen on the LSAT, there are both stronger and weaker arguments. Weak arguments are not logically valid, but not all strong arguments are logically valid either. There might be very strong evidence for a conclusion without a guarantee. We would then say the argument is strong but not valid. Consider the following three examples.

    Weak Argument

    Jim's car broke down.
    Chevy Novas break down all the time.
    Therefore, Jim must drive a Chevy Nova.

We could describe this argument as both invalid and lacking logical force.

    Stronger Argument

    I've eaten cherry pie 100 times.
    Every time I've eaten cherry pie, the next morning I have a stomachache.
    I just ate cherry pie.
    Therefore, tomorrow morning I will have a stomachache.

This argument has much more logical force. There is excellent evidence that tomorrow morning I will have a stomachache. However, it falls short of logical validity. The truth of the premises here does not in and of itself guarantee that I will have a stomachache tomorrow—perhaps this time things will be different!—but these premises do provide very strong evidence for our conclusion.

    Logically Valid Argument

    If I fall into a pool of water, I will be wet.
    I just fell into a pool of water.
    Therefore, I am wet.

This argument has an even higher degree of logical force: it is logically valid. The truth of the premises guarantees the truth of the conclusion. This is as strong as arguments get on the LSAT.

To recap, there are:

  1. Weak Arguments: These lack logical force and are invalid.
  2. Stronger Arguments: These have greater logical force but are not necessarily valid.
  3. Valid Arguments: These have the greatest possible logical force.

I hope this helps.
Leslie Wallace
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Got it! Thanks so much!
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Not necessarily, in some cases when the stimulus use premises with strong logical force to support a conclusion that uses weak logical force, the argument is valid. For example, if I say all A’s are B’s. You can validly conclude that some A’s are B’s. But if I simply tell you that some A’s are B’s, your cannot validly conclude that all A’s are B’s. Notice the difference in logical force between the word “some” and “all”.
Alex Bodaken
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Thanks for the comment! You are certainly correct: an argument with strong logical premises that support a weaker conclusion can still be logically valid. I don't read that as conflicting with anything that Jonathan has said above.

Hope that helps!