Piggy-back assumption:

Evidential Fallacy
Definition Example
When one assumption is connected to a second assumption in a non-contingent way, then the 1st assumption is deemed justified by evidence for the second assumption. I’ve told you that I’m a CIA agent from Boston. Here’s my driver’s license showing a Boston address. That should make it clear that I work for the CIA.
Also known as: stacked assumptions
  Notes
This fallacy is true only where there is a non-contingent relationship. However, if you were to claim you had been married and produced an official document of your divorce, that constitutes evidence for both the divorce and a necessary prior marriage. To a softer degree, claiming that you met the president, then producing a photo of you together with the president’s wife would constitute evidence for your claim you met the president.


This is related to the all-or-nothing fallacy.

Case Study One

Some Christians suggest that archeological evidence of sites mentioned in the Bible is evidence for the biblical miracles mentioned in the same passage as is the site. Uncovering historical evidence for the existence of St. Nicholas does not lend credibilty to flying reindeer.


Case Study Two

Some people argue that evidence for government impropriety on issue X implies that there is also impropriety on issue Y. While inductive evidence derived from assessing habitual behavior warrants a measured degree of speculation, it does not warrant confidence incommensurate to that evidence.


Keep in mind that a fallacious argument does not entail an erroneous position.


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