The Logical Two Step
Evaluate an argument’s reasoning and its premises separately. For a deductive argument, check whether the conclusion follows, then whether the premises are true. A valid argument with a false premise is unsound; an inductive argument needs an appropriately qualified conclusion.
THE IDEA
When evaluating a deductive argument, separate the reasoning from the truth of its premises:
Step 1: Make sure the conclusion follows from the premise(s).
Step 2: Make sure all of the premises in the argument are true.
Writing the premises and conclusion separately makes the reasoning easier to inspect. Consider:
- Computers can calculate things.
- The human mind can calculate things, too.
- Therefore, the mind is a computer.
The source invokes roughly 60 years of artificial-intelligence research to explain why this comparison might appeal to readers. But a shared ability does not establish identity, as the same argument form makes clear:
- Birds can fly.
- Airplanes can fly, too.
- Therefore, an airplane is a bird.
Sharing the ability to fly does not make an airplane a bird. Likewise, the ability to calculate does not establish that a mind is a computer. These arguments commit the fallacy of the undistributed middle term.
A valid form is not enough, however. The premises also need to be true. This argument is valid:
- Our sun is a star.
- All stars are cube‐shaped.
- Therefore, our sun is cube‐shaped.
The premises entail the conclusion “Therefore, our sun is cube‐shaped.” The argument is unsound because the premise about the shape of stars is false. The conclusion happens to be false too, but a false premise does not guarantee that outcome in every argument. Now consider a different kind of inference:
- In over 1 million controlled studies, it was shown consistently that people lost 10% of their body weight in a month on a 5,000‐calorie‐a‐day diet.
- You are starting that same diet at the beginning of next month.
- Therefore, all things being equal, you too will likely lose 10% of your body weight in a month on a 5,000‐calorie‐a‐day diet.
The study claim was invented for the example. Even if such evidence existed, the inference to an individual’s likely result would be inductive, requiring attention to whether the findings applied to that person. As written, the fabricated premise supplies no evidence for the prediction; it does not prove that the predicted outcome is false.
When to use it
Evaluating claims in advertisements or political speeches by checking if the conclusion logically follows and if the premises are factual.
Analyzing scientific arguments in research papers or news articles to distinguish between valid reasoning and fallacious claims.
Teaching students or colleagues how to construct and critique arguments by emphasizing the two-step process.
Improving personal decision-making by testing the logical structure and factual basis of arguments for or against a course of action.
FROM THEORY TO EVERYDAY LIFE
In practice
Mind as Computer Fallacy
- Computers can calculate things.
- The human mind can calculate things, too.
- Therefore, the mind is a computer. This argument commits the fallacy of the undistributed middle term; the conclusion does not logically follow.
Airplane as Bird Analogy
- Birds can fly.
- Airplanes can fly, too.
- Therefore, an airplane is a bird. This argument has the same form as the mind-computer argument, demonstrating the fallacy clearly.
Valid but Unsound: Cube-shaped Sun
- Our sun is a star.
- All stars are cube-shaped.
- Therefore, our sun is cube-shaped. The conclusion follows logically (no fallacy), but premise 2 is false, making the argument unsound.
Invented Evidence: 5,000-Calorie Diet
- In over 1 million controlled studies, people lost 10% of body weight on a 5,000-calorie-a-day diet.
- You are starting that diet.
- Therefore, you will likely lose 10% of body weight. The study claim in premise 1 was invented. It therefore provides no support for the prediction. The inference to a likely individual outcome is inductive, rather than deductively valid.
A MOMENT TO REFLECT
Put it to work.
Think of an argument you recently encountered (e.g., in news, social media, or conversation). Apply the two-step process: Does the conclusion follow from the premises? Are all premises true?
Why is it important to check the truth of premises even if an argument is logically valid? Can you think of a real-world example where a valid argument misled people because of false premises?
How does the ability to spot fallacies help you in everyday reasoning? Try to identify a fallacy in a persuasive message you see today.
CONTINUE THE CONVERSATION
Source texts
- Bad Arguments 100 of the Most Important Fallacies in Western Philosophy (Steven Barbone, Michael Bruce)