In a categorical proposition, stating that “some A are B” does not allow us to conclude that “some A are not B”. That contrast is not warranted by the premise.
Example
“Some stars have planets”
“Therefore, some stars do not have planets”
(The conclusion may be true or false, but it does not follow.)
Applied example (political)
“Some laws improve the economy; therefore some laws worsen it.” (It does not follow from the quantifier.)
Applied example (mystical)
“Some people feel relief with reiki; therefore some people do not.” (It does not follow from the premise.)
Why it is fallacious
- The quantifier “some” does not imply contrast.
- New information is introduced without support.
- It confuses possibility with valid inference.
How to spot it
- A jump from “some” to “some not” without evidence.
- An assumed balance that is not demonstrated.
- A negation inferred without data.
How to respond
- Ask for additional evidence that establishes the contrast.
- Point out that the premise does not force that conclusion.
- Offer counterexamples where all A are B.