‘Now it is also the case that there can be nothing intermediate to an assertion and a denial. We must either assert or deny any single predicate of any single subject. The quickest way to show this is by defining truth and falsity. Well, falsity is the assertion that that which is is not or that that which is not is and truth is the assertion that that which is is and that that which is not is not. Thus anyone who asserts anything to be or not to be is either telling the truth or telling a falsehood. On the other hand, neither that which is is said either not to be or to be nor is that which is not.
And if there were an intermediate of contradictory statements, then it would either be like grey between black and white or like the non-man-non-horse between man and horse.’
Aristotle The Metaphysics, Gamma 7 1011b, Trans. and Introduction by Hugh Lawson-Tancred, Penguin, London, 2004, 107
‘I want to tell you of one more thing that I see to be marvellous above other things. …since all things are singular, they are both similar, because they are singular, and dissimilar, because they are singular; (and they are not similar, because they are singular), and not dissimilar, because they are singular. A corresponding point holds regarding same and different, equal and unequal, singular and plural, one and many, even and odd, concordant and discordant, and the likes, although this (claim) seems absurd to the philosophers who adhere – even in theological matters – to the principle that each thing either is or is not (the case).’
Nicholas of Cusa, De Venatione Sapientiae (On the Pursuit of Wisdom), Nicholas of Cusa: Metaphysical Speculations, Six Latin Texts Translated into English, Trans., Jasper Hopkins, The Arthur J. Banning Press, Minneapolis, Minnesota, 1998, 1320-21