The reply is:

This may be answered in several ways. First, taking precautions concerning water on the basis of mere doubt is neither recommended nor prescribed. It is not even recommended to inquire into such matters. Rather, what is prescribed is to proceed on the presumption that the established state continues. If evidence of impurity emerges, we deem the water impure; otherwise, avoiding its use merely because it might be impure is not recommended. If, however, there is an apparent indication of impurity, that is a different matter.

The decisive evidence is that the Prophet (peace and blessings be upon him), the Companions and their successors consistently performed ablution, bathed and drank using water from vessels, small buckets, basins and other receptacles, despite this possibility. Indeed, no possibility unsupported by an indication recognized by Islamic law is to be given weight.

This is because prohibited things fall into two categories: those prohibited because of their inherent qualities, and those prohibited because of how they are acquired. Things prohibited because of how they are acquired include what is obtained through injustice, usury and gambling. Things prohibited because of their inherent qualities include carrion, blood, pork and animals slaughtered with the invocation of a name other than Allah’s. The former are more strictly prohibited, and scrupulous avoidance of them is well known. Thus, the early generations guarded against doubtful food and clothing arising from unlawful earnings.

As for the second category, such things are prohibited only because of the foulness inherent in them. Allah has permitted us the food of the People of the Book, even though they might not slaughter it in the manner prescribed by Islamic law, or might invoke a name other than Allah’s over it. If we know that they invoked a name other than Allah’s over it, it is prohibited