two kinds:

The first consists of negations whose scope we can delimit and fully comprehend. Examples include our knowledge that there are not two suns or two moons aloft in the sky; that we have only one direction of prayer; and that no prophet will come after Muhammad. Indeed, we know that there is no deity worthy of worship except Allah; that whatever is not between the two covers is not part of the Quran; and that no fasting has been prescribed except that of the month of Ramadan. Other examples include a person’s knowledge that there are no dirhams in … (1) before (2), that he has not changed, that he has not been fed, and that he did not sleep last night. The examples are too numerous to enumerate. All these are negations known with certainty, demonstrating the error of anyone who declares without qualification that testimony concerning a negative fact is inadmissible.

The second kind:

This concerns matters whose negation or nonexistence cannot be known with certainty. Some of these are nevertheless strongly believed and judged highly probable, while others are not. When we find a ruling contingent upon a negative claim of this second kind, what is required is that we judge the negation to hold and believe it highly probable. Reasoning from the presumption that an established state continues, from the absence of evidence restricting a general statement, and from the absence of any reason to interpret an expression figuratively all belongs to this category.

Thus, when we have searched and carefully examined the evidence indicating that these substances are ritually impure—a question people have discussed for hundreds of years—and have found only the familiar proofs, we can testify firmly, to the best of our knowledge, that there is no other evidence on this matter. We therefore say that the present argument can be fully established only by overturning the evidence adduced for ritual impurity