Just read this on another blog, and have to put it here so I don’t forget it. I refer to this concept quite frequently, but didn’t realize that it was put together so succinctly already.

ARGUMENTUM AD IGNORANTIAM

An argument that a proposition is true because it has not been shown to be false, or vice versa. Ad ignorantium arguments are also known as “appeals to ignorance.” This fallacy has two forms:

1. P is true, because it has not been proven false.
2. P is false, because it has not been proven true.