
“Minus times minus equals plus,
The reason for this we need not discuss.”
As stated in "The Poet Himself" http://query.nytimes.com/gst/fullpage.html?res=9F01E6D81539F937A35753C1A967948260 by Paul Fussell, in The New York Times (4 October 1981), these lines were a "math mnemonic" which Auden "had to memorize as a child."
Misattributed