Principle of Purity
The Principle of Purity (Arabic: قاعدة الطهارة) states that all things are to be considered ritually pure until proven otherwise. This is a universally accepted principle amongst religious jurists. Many narrations are offered in support of it, such as the narration from Imam Jafar al-Sadiq (as):
كل شيء نظيف حتى تعلم أنه قذر ، فإذا علمت فقد قذر ، ومالم تعلم فليس عليك
Everything is clean until you know that it is unclean. If you know, then it is to be considered unclean; otherwise, there is no obligation upon you.
It should be remembered, however, that "knowing" does not necessarily mean actual knowledge. If a thing was ritually impure before, and then one doubts that it is still pure, the principle of Istishab would apply, and the thing would still be considered to be ritually impure.