—evidence that necessarily entails his deceit—and by the absence of the concomitants of truth, whose disappearance necessarily means that truthfulness itself is absent. Allah knows best.

A matter can sometimes be known by what directly indicates its reality, and at other times by what indicates that its opposite is absent. This latter method is what logicians call qiyās al-khulf (proof by contradiction): when an issue is restricted to two possibilities, the establishment of one automatically negates the other, and the negation of one automatically establishes the other. Accordingly, a claimant to prophethood is either truthful or a liar; each alternative has concomitants whose absence shows that the alternative is absent, and sufficient conditions whose presence shows that the alternative is present.

Thus, whatever serves as a proof of something necessarily entails it—such as the tokens and evidences of prophethood, the signs of Lordship, the proofs of legal rulings, and so forth—whereas the non-existence of something is known through what necessarily negates it, namely the absence of its concomitants. Consider, for example, attributing truthfulness to a liar; one would say:
‘If he were truthful, he would possess the traits that characterize truthful people.’

Likewise, in attributing falsehood to a truthful person, one would say:
‘If he were a liar, he would display the traits that characterize liars.’

The states of the truthful prophets and of the lying pretenders are both well known. Therefore, the absence of the concomitants of falsehood is evidence of his truthfulness, just as the presence of that which necessitates truthfulness is evidence of his truthfulness. Conversely, a liar is proven to be lying by what necessarily entails his lying and by the absence of the concomitants of truthfulness—and so it is with all other matters.