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[Source] [Slide 20:] An area has been shaded twice. So, we haven’t diagrammed the conclusion. The argument form is invalid.

[Slide 27 of 27:] No syllogism whose diagram places an X on the line or results in double-shading is valid.

I already understand, and so ask not about, the particular example in Slide 20. Instead, what are the big picture and intuition behind the title of this question?
Please do not answer with formal proofs or arguments.

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The example [slide 20] is not clear to me ...

According to the Venn diagram, we are left with an "unshaded" area at the intersection of S and P.

Thus, accordind to rule of slide 7 :

if by diagramming the premises you have not diagrammed the conclusion, the argument is invalid,

this seems enough to conclude with the invalidity of the argument.

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