If one argues:
I have seen only white swans, therefore there are no black swans.
What would this fallacy be called?
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2This is so common a sort of error that it has multiple different possible origins. It would be labeled depending upon how you actually did the decision making: Induction without a mechanism, statistics from an ad-hoc sample, premature generalization, plain old lack of imagination...– user9166Nov 16 '19 at 15:25
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This also reminds me of Falsifiability [en.wikipedia.org/wiki/Falsifiability] introduced by Karl Popper.– PenumbraBrahNov 26 '19 at 17:26
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This would be a straight-forward case of "absence of evidence is not evidence of absence" or what is called argument from ignorance. As the article states:
This represents a type of false dichotomy in that it excludes the possibility that there may have been an insufficient investigation to prove that the proposition is either true or false.
Hence it is an inference with an enthymeme:
P1 I have seen only white swans.
(P2 What I see is all there is)
C Therefore there are no black swans.
This also touches on the concept of paradoxes of confirmation such as the raven paradox. This scenario has been highlighted by Nassim Nicholas Taleb recently in his Black swan theory. See related SE Post here.
REFERENCES
Damer, T. Edward. Attacking Faulty Reasoning
Bennett, Bo. Logically Fallacious: The Ultimate Collection of Over 300 Logical Fallacies
RESPONSES
Could this also be an example of [a]ffirming a disjunct? – Himmators
The structure is very closely related, but it is not an example because there would be a slight difference in the structure of the inference. We need to have a disjunction present. Also notice we've shifted our language to conceal the empirical nature of our propositions.
P1 Swans are (either) white or black.
P2 All swans are white
C Therefore there are no black swans.
In this argument, notice we have replaced our enthymeme with an explicit assertion of two possibilities.