Russell's Teapot is an argument first formulated by the philosopher Bertrand Russell as a reductio ad absurdum of the notion that in an argument about the possible existence of an unseen entity, the burden of proof is upon the skeptic.
|“||If I were to suggest that between the Earth and Mars there is a china teapot revolving about the sun in an elliptical orbit, nobody would be able to disprove my assertion provided I were careful to add that the teapot is too small to be revealed even by our most powerful telescopes. But if I were to go on to say that, since my assertion cannot be disproved, it is an intolerable presumption on the part of human reason to doubt it, I should rightly be thought to be talking nonsense. If, however, the existence of such a teapot were affirmed in ancient books, taught as the sacred truth every Sunday, and instilled into the minds of children at school, hesitation to believe in its existence would become a mark of eccentricity and entitle the doubter to the attentions of the psychiatrist in an enlightened age or of the Inquisitor in an earlier time.||”|
The fallacy in the argument is that there is in fact nothing absurd about believing the teapot to be there, if those "ancient books" were written by an ancient astronaut or other being who placed the teapot there. The argument presumes that such is not the case, so it presumes what it sets out to prove, and is thus a circular argument.
That is, the argument is based on the presumption that there is no valid reason, beyond widespread belief, to believe that the teapot exists. But if the validity of those ancient books could be established, there is indeed reason to believe that the teapot exists, and thus the presumption in the argument is false.
Charles Darwin postulated that species adapt over time to the environment around them. Since Darwin makes the claim, the burden of proof rests on his shoulders. However, supporters of Darwin often claim that evolution is correct because it is unfalsifiable.[Who says?]