Argument from incredulity
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The argument from incredulity is a logical fallacy that occurs when someone decides that something did not happen, because they cannot personally understand how it could happen.
 Alternate names
- argument by lack of imagination
- argument from personal incredulity
- argumentum ad ignorantiam
P1: One can't imagine how X could be true.
P2: (unstated) If X is true, then one could imagine how X could be true.
C1: X is false.
The fallacy lies in the unstated premise. If a state of affairs is impossible to imagine, it doesn't follow that it is false; it may only mean that imagination is limited. Moreover, if no one has yet managed to imagine how a state of affairs is possible, it doesn't follow that no one will ever be able to.
If there is positive evidence for the conclusion, then of course we have other reasons for accepting it, but a lack of proof it is false is not proof it is true. (Where 'proof' means the cogency of evidence that compels acceptance of a truth, or the process of establishing the validity of a statement by derivation from other statements in accordance with principles of reasoning.)
Forbidding this type of reasoning is not the same as a reasonable presumption (such as the presumption of 'No guilt' in court) like this:
- Any proposition like "There is X" is reasonably presumed false (not argued false, just presumed false, taken for granted) unless proven true.
This is a basic principle of logic, since the burden of proof can never be shifted to the negation in any case,
- One cannot prove that the Loch Ness Monster is certainly a fake. The creature continues to evade all scientific endeavours to locate it, and no conclusive proof exists indicating that sighting were faked. Therefore, the Loch Ness Monster is real.
- "Scientists are at a loss to explain the evolution of the platypus by Darwinian evolution. (Unwritten assumption: If it was of a Darwinian origin, scientists should know how it happened). Therefore, it didn't evolve."
As an example, creationists incessantly use some difficult-to-explain facet of biology as "proof" of a creator. The problem is that, though there is no non-design explanation for how precisely a certain organ could have evolved at the moment, one may be discovered in the future. Contrary to the instincts of many creationists, lack of an explanation does not justify confecting whatever explanation one would prefer. The inexplicable is just that, and does not justify speculation as proof.
Sometimes creationists compute the astronomical odds against a molecule having a certain structure from the simple probability of n atoms arranging themselves so. They gloss over the fact that chemical laws trim most of the extraneous possibilities away. For instance, there are many ways to theoretically arrange hydrogen atoms and oxygen atoms in a molecule, but in reality, most of what forms is H2O. Note that the creationist's fundamental error is not his ignorance of this fact, but his assumption that there is nothing more to know.
A "Christian Cynic" writes:
P1: I cannot imagine how human beings, as complex as they are, evolved from a unicellular organism.
Of course, the missing premise is:
C1: If common descent were true, I would be able to imagine how human beings evolved from a unicellular organism.
and with its inclusion the argument is formally valid. Its soundness, therefore, depends on the veracity of the missing premise, and most people (myself included) would dispute it based on the fact that current evidence should make it much easier to imagine the evolution from unicellular organism to complex multicellular organism by the examination of transitional forms.
Strangely, the cynic never attempts to imagine how complex it would be for God to tinker with the genes of innumerable animals and achieve the desired result.
"Scientists cannot explain this" (meaning, of course, "as far as I know, science can't explain this"). This variation contains the unwritten assumption that scientists are superhuman geniuses and should be able to understand everything unless they are missing an assumption. This undue veneration of scientists is a form of scientism, or using science as an ersatz religion. On top of that, it is simply not true in many cases - scientists do have an explanation, and the speaker just doesn't know it.
 Personal incredulity
Another form, the argument from personal incredulity, takes the form "I can't believe P, therefore not-P." Merely because one cannot believe that, for example, homeopathy is no more than a placebo does not magically make such treatment effective. Clinical trials are deliberately designed in such a way that an individual personal experience is not important compared to data in aggregate. Human beings have extremely advanced pattern recognition skills, to the extent that they are objectively poor judges of probability.
- "This is unexplainable" (meaning, of course, "I can't explain this"). This is the argument from personal incredulity, and it contains the (usually unwritten) assumption that the speaker is a superhuman genius who should be able to understand everything -unless he or she is missing an assumption. So the superhuman genius concludes that some assumption ('God did it', 'aliens did it', 'psi was involved' or whichever) is true, because it makes things easier to understand. For example:
- "There is no way I can explain how the human mind really works using conventional physics. (Unwritten assumption: If the brain really was governed by simple physics, I should be able to understand it). Therefore, it must be tapping into the computational power of the quantum universe."
 General incredulity
Sometimes argument from incredulity is applied to epistemological statements, taking the form "One can't imagine how one could know whether P or not-P, therefore it is unknowable whether P or not-P." This is employed by some (though not all) strong agnostics who say it is unknowable whether gods exist. The argument in this case is, "No one has thought of a way to determine whether there are gods, so there is no way." The implied major premise, "If there were such a way, someone would have thought of it," is disputable.
 See also
- Argument from ignorance
- Argument from silence
- Sherlock Holmes fallacy
- God of the gaps
- Argument from design
- Argument from beauty
- divine fallacy (argument from incredulity), Skeptic's Dictionary
- Bruce Thompson 
- Fallacy Files 
- Internet Encyclopedia of Philosophy 
- Objectivism 
- Wikipedia 
- Skeptic's Dictionary 
- Stephen Downes 
- Julian Baggini, Bad Moves: Arguments from incredulity 
- Video Explanation 
 Further reading
- Drummond, Henry, 1904. The Lowell Lectures on the Ascent of Man , Glasgow: Robert Maclehose and Co. Ltd., (Chpt. X has perhaps the first reference to God of the Gaps.)