No amount of observations of white swans can allow the inference that all swans are white, but the observation of a single black swan is sufficient to refute that conclusion.
David HumeThe difference between a man who is led by opinion or emotion and one who is led by reason. The former, whether he will or not, performs things of which he is entirely ignorant; the latter is subordinate to no one, and only does those things which he knows to be of primary importance in his life, and which on that account he desires the most; and therefore I call the former a slave, but the latter free.
David HumeAll that belongs to human understanding, in this deep ignorance and obscurity, is to be sceptical, or at least cautious, and not to admit of any hypothesis whatever, much less of any which is supported by no appearance of probability.
David Hume