Not-being has nothing in itself whereby it can be known
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17611/pg17611-images.html

Context

“It may, however, be said that being also is in things and in the intellect, as is the true; although truth is primarily in the intellect, while being is primarily in things; and this is so because truth and being differ in idea. Reply Obj. 2: Not-being has nothing in itself whereby it can be known; yet it is known in so far as the intellect renders it knowable. Hence the true is based on being, inasmuch as not-being is a kind of logical being, apprehended, that is, by reason. Reply Obj. 3: When it is said that being cannot be apprehended except under the notion of the true, this can be understood in two ways.” source