If on account of not knowing the Islamic ruling one does not know whether an object is impure or pure – for example, he does not know whether the droppings of a mouse are pure or not – he must inquire about the ruling.
However, if despite knowing the ruling one doubts whether an object is pure or not – for example, he doubts whether something is blood, or he does not know whether it is the blood of a mosquito or the blood of a human being – then in these cases, the object is pure and it is not necessary for him to investigate or ask about it.