Categorical syllogism with an affirmative conclusion based on a negative premise

A categorical syllogism with an affirmative conclusion from a negative premise is invalid. If a premise is negative, the conclusion must be negative or it does not follow.

Example

“All dogs are animals.
Some animals are not herbivores.
Therefore, some dogs are herbivores.”
(The affirmative conclusion does not follow.)

Applied example (political)

“All judges are officials. Some officials are not honest. Therefore some judges are honest.” (The affirmative conclusion does not follow.)

Applied example (mystical)

“All healers are people. Some people are not spiritual. Therefore some healers are spiritual.” (The form is invalid.)

Why it is fallacious

• A negative premise restricts the conclusion.
• It asserts something not guaranteed.
• It mixes inclusion with exclusion without justification.

How to spot it

• At least one negative premise.
• An affirmative conclusion with no valid link.
• A sign change without basis.

How to respond

• Require a negative conclusion when a premise is negative.
• Check whether the middle term connects validly.
• Ask for additional evidence.