The fallacy of double counting occurs when an arguer sums up items in a way that includes certain item(s) more than once, thus arriving at a larger final number than should be the case. This usually happens because of dividing the total set into sub-categories, where at least one item gets counted in two or more sub-categories, resulting in a wrongly higher tally.

Here is an example:

"Wow, I guess there are 40,000 books in the Wilson Humanities Library. Because in looking across the four main category pages on their website, I see that they have 15,000 titles in Art & Literature, 10,000 in History & Biography, 8,000 in Philosophy and Social Theory, and 7,000 in Analysis and Criticism -- which adds up to 40,000."

The problem here is that many books belong to more than one category. For example, a book entitled Stylistic Developments in Renaissance Painting is likely indexed in the library system as both a History and an Art title. The simplistic mathematical sum asserted above is therefore very likely to be a significant overstatement.

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