من العلوم المهمة التي يمکن أن تتصدى للبدع علم أصو ل الفقه، فإنه يضبط االستدالل، ويحدد مصادره، وبه يتبين الصحيح من عدمه، ومن العلماء الذين استعملوا هذا العلم ابن تيمية، ومما استعمله قاعدة: عدم التکليف بالمحال في رد کثير من البدع؛ لکونها األصل الذي يعتمد عليه، وبسبب عدم تحرير القاعدة، ومعرفة لوازمها انبنى عليه مسائل کثيرة؛ لذلک أکثر ابن تيمية من استعمالها بتحرير وتحقيق. Abstract From the important sciences that make it possible to oppose innovations is the science of Usool Al-Fiqh, for indeed it regulates the deriving of evidences and clarifies it's sources. By it, the authentic are clarified from other than it. And from the scholars who utilised this science was Ibn Taymiyyah and from what he used was the principle: not being assigned with the impossible to refute many of the innovations, for it is a foundation referred to. And because the principle hasn't been clarified and what it necessitates is not known, many rulings have stemmed from it. For this reason Ibn Taymiyyah used it frequently and clarified it with precision.