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The obvious way to disprove an impossibility conjecture is by providing a single counterexample. For example, Euler proposed that at least n different n th powers were necessary to sum to yet another n th power. The conjecture was disproved in 1966, with a counterexample involving a count of only four different 5th powers summing to another ...
For example, some unicellular organisms have genomes much larger than that of humans. Cole's paradox: Even a tiny fecundity advantage of one additional offspring would favor the evolution of semelparity. Gray's paradox: Despite their relatively small muscle mass, dolphins can swim at high speeds and obtain large accelerations.
Such a proof is again a refutation by contradiction. A typical example is the proof of the proposition "there is no smallest positive rational number": assume there is a smallest positive rational number q and derive a contradiction by observing that q / 2 is even smaller than q and still positive.
Proof of impossibility, mathematics; Russell's teapot, an analogy: inability to disprove does not prove; Sometimes it is mistaken for an argument from ignorance, which is non-proof and a logical fallacy
Zeno's arguments may then be early examples of a method of proof called reductio ad absurdum, also known as proof by contradiction. Thus Plato has Zeno say the purpose of the paradoxes "is to show that their hypothesis that existences are many, if properly followed up, leads to still more absurd results than the hypothesis that they are one."
When this recursion creates a metaphysical impossibility through contradiction, the regress or circularity is vicious. Again, the liar paradox is an instructive example: "This statement is false"—if the statement is true, then the statement is false, thereby making the statement true, thereby making the statement false, and so on. [15] [18]
Impossibility of performance, in contract law an excuse for non-performance of a contract Impossibility defense , a criminal defense for a crime that was legally impossible to commit Proof of impossibility , in mathematics a proof that demonstrates that a particular problem cannot be solved
A system will be said to be inconsistent if it yields the assertion of the unmodified variable p [S in the Newman and Nagel examples]. In other words, the notion of "contradiction" can be dispensed when constructing a proof of consistency; what replaces it is the notion of "mutually exclusive and exhaustive" classes.