Ruling 220
If the body, clothing, or something else like a utensil or carpet becomes impure, and if it is in the possession of a Muslim who is bāligh – or a non-bāligh mumayyiz child who is able to discern between what is pure and what is impure – and that Muslim moves out of sight [with the impure object], then in the event that one deems it rationally probable that he has washed it, it is pure. This rule also applies to a non-bāligh child who is not mumayyiz as the child’s affairs are looked after by such a [Muslim] person entrusted with its affairs. Furthermore, things that a person cannot see because of a lack of light or being blind fall under the rule of becoming absent. Therefore, if the body or clothing of a Muslim becomes impure and someone does not see it being purified due to a lack of light or being blind, then in the event that he deems it rationally probable that it has been washed, it is ruled to be pure.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)