Other terms and/or Related ConceptsSelf-refuting idea; Self-refuting sentence; Self-contradiction; Tautology; Begging the Question; Circular reasoning
DescriptionA Self-Defeating Argument is one that is internally inconsistent (self-contradictory) and as such, by definition is false (and therefore meaningless). The proponent has inadvertently set up a paradoxical circular argument; if the conclusion is accepted as true, the premises must be false (which therefore means the conclusion must actually be false, which then could mean the premises are true... ad infinitum).
A classic example is the liar paradox. Consider the following:
If we accept the first sentence, then we must conclude the second sentence is false. However, in doing this, we invalidate the "truthfulness" of the first sentence, but if that’s the case...
The following sentence is false. The preceding sentence is true.
ExampleDani Nigher is arguing with her friend Con Senses about climate change.
"Global warming just can't happen. Think about it. They say the overall temperature of the Earth's oceans will increase. What happens to the volume of water when it gets hotter? It increases! An increase in ocean levels will mean an increase in ocean volume and as such, ocean surface area. But, if the ocean surface area increases, we'll also get, as a consequence, more evaporation.
More evaporation means heat gets taken away from the surface, and more importantly, more cloud cover, which will stop sunlight reaching the surface of the Earth. More light will be reflected into space. That is, the increase in evaporation will actually cause cooling, and cancel out global warming from the greenhouse effect!"
CommentIgnoring all other flaws on Dani's argument, we will just focus on her logic. If we believe the conclusion, that excessive evaporation will cancel out warming by greenhouse gases, then how did the ocean surface area increase? (A prediction of global warming is the melting of polar ice caps, and thermal expansion of the oceans, will cause a rise in ocean levels.) The conclusion contradicts the premise, so is invalid. By definition the premise must be accepted as true for the conclusion to be valid. But if the conclusion contradicts the premise, we have a Self-Defeating Argument.
Now, of course, giving Dani the benefit of the doubt, we could rephrase her idea and propose a reasonable, non-contradictory hypothesis. For example, the increase in ocean surface area could damp climate change causing a lag in warming. Or, the increase in surface area could be involved in a cycle, that means periodically the temperature goes up, then goes down, then goes up.... and so on. Whatever the case, it won't be a simple cancellation.
Without having consulted any literature on climate change, or bothering to talk to even a single climate scientist, one still feels fairly safe in presuming variables, such as the increase in surface area of oceans and consequent evaporation, is something that (among thousands of other variables) scientists do account for.