obversion

noun

1. (countable, uncountable)The act of turning toward or downward.
2. (countable, uncountable)An immediate inference that denies the opposite of something previously affirmed.

   “All men are mortal, so, by obversion, no men are immortal.”

   “From 'warmth is agreeable' we can affirm, by formal obversion, 'warmth is not disagreeable, and not indifferent.' We cannot affirm, without an examination of the subject matter, 'cold is disagreeable.'”