The quotation you have is explained here The Illusions of Postmodernism - Google Book Search (Page 29, 3rd paragraph). The reason I've provided it, defining "negative truth" proves to be somewhat complicated. Here's my best try,
For example, the statement the cup is broken has a certain truth value, whereas the statement the cup is not broken has neither a truth of false value, as it applies to nothing. It's a negative truth.
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