No A are B. All B are C. So, no A are C. a) Is this form a fallacy?

Question

 No A are B. All B are C. So, no A are C. a)

 

Is this form a fallacy?

 

Is it a formal or an informal fallacy?

Answer 1

This is indeed a fallacy. Think about this:

 

Let A = pentagons, B = quadrilaterals, and C = polygons. Then:

 

No pentagons are quadrilaterals.

All quadrilaterals are polygons.

So, no pentagons are polygons.

 

The first two statements are true. So, if the argument is valid, the conclusion should also be true. But it is not, since all pentagons are actually polygons.

 

This is a formal fallacy, since something is wrong with the formal statement of it. That's clear from our example, because two true statements should result in a valid conclusion, but since the form is wrong, they do not.