Accordingly, someone who does not grasp the impossibility from the term “cause” may use this to establish it. As for someone who understands the impossibility from the term “cause,” as most people of sound natural disposition do, there is no need for this.

For one thing to be evidence of another means that establishing the former entails establishing the latter. When two things necessarily accompany one another, either can serve as evidence of the other. People ordinarily infer what is obscure from what is evident. Yet what is evident and what is obscure are relative: something may be evident to one person but not to another, and something evident to a person at one time may be obscure to that same person at another.

It is therefore possible to infer this from that, and that from this, provided that this is more evident than that on one occasion, and that is more evident than this on another—whether for two different people or in two different states.

Anyone who grasps these points will be freed from many doubts arising from the objections people raise against definitions and proofs. It may be said that they offer no benefit and are unnecessary, and this is correct. It may also be said, “Rather, they are beneficial,” and this too is correct.

But someone who confines knowledge to one particular route—a particular example and a particular proof—is greatly mistaken. Likewise, someone who says that another person’s definition and proof can never be useful is greatly mistaken.

Similarly, those who made rational reflection obligatory and said, “Knowledge can only ever be attained through it,” were mistaken. Those who said, “There is never any need for it; rather, knowledge is always self-evident to everyone in every circumstance,” were also mistaken. Rather, knowledge