Qiyas(analogical comparison) of the opposite is to negate a ruling due to negation of its `illah (legal reason) and effective cause. Qiyas is of two kinds:
1) Direct qiyas wherein the ruing of the far` (new case) is affirmed due to affirmation of the `illah (legal reason) of the asl (original case) therein.
2) Opposite qiyas wherein the ruing of the far` (new case) is negated due to negation of the `illah (legal reason) of the ruling therein.
[I`lam al-Muwaqqi`in, Ibn al-Qayyim (1/160)]
قياس العكس وهو نفي الحكم لنفي علته وموجبه