Affirmative Conclusion from a Negative Premiss

Type: Syllogistic Fallacy

Form:

Any form of categorical syllogism with an affirmative conclusion and at least one negative premiss.

Example Counter-Example
All judges are politicians.
Some lawyers are not judges.
Therefore, some lawyers are politicians.
All whales are mammals.
Some fish are not whales.
Therefore, some fish are mammals.
Venn diagram

Venn Diagram:

This diagram represents both the Example and Counter-Example, and shows that neither is valid, since the conclusion, "Some S is P", is not shown to be true.

Syllogistic Rule Violated:

All validating forms of categorical syllogism which have one negative premiss also have a negative conclusion.

Source:

Irving Copi & Carl Cohen, Introduction to Logic (10th Edition), (Prentice Hall, 1998), pp. 277-8.


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