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Exceptive syllogism

al-qiyās al-istithnāʾī

القِيَاسُ الاسْتِثْنَائِيّ

An exceptive syllogism (istithnāʾī) is one in which the conclusion or its contradictory is stated in the premises.

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Explanation

It joins a conditional premise to an exceptive one, introduced by “but”: “If the sun has risen, it is day; but the sun has risen; so it is day.”

Also written as: istithnai, hypothetical syllogism.

In the text

وَهُوَ إِمَّا اقْتِرَانِيٌّ كَقَوْلِنَا: كُلُّ جِسْمٍ مُؤَلَّفٌ، وَكُلُّ مُؤَلَّفٍ حَادِثٌ، فَكُلُّ جِسْمٍ حَادِثٌ؛ وَإِمَّا اسْتِثْنَائِيٌّ كَقَوْلِنَا: إِنْ كَانَتِ الشَّمْسُ طَالِعَةً فَالنَّهَارُ مَوْجُودٌ، لَكِنِ النَّهَارُ لَيْسَ بِمَوْجُودٍ، فَالشَّمْسُ لَيْسَتْ بِطَالِعَةٍ.It is either conjunctive, as in “Every body is composite, and everything composite came to be, so every body came to be”; or exceptive, as in “If the sun has risen, it is day; but it is not day, so the sun has not risen.”وَأَمَّا الْقِيَاسُ الِاسْتِثْنَائِيُّ فَالشَّرْطِيَّةُ الْمَوْضُوعَةُ فِيهِ إِنْ كَانَتْ مُتَّصِلَةً، فَاسْتِثْنَاءُ عَيْنِ الْمُقَدَّمِ يُنْتِجُ عَيْنَ التَّالِي، كَقَوْلِنَا: إِنْ كَانَ هَذَا إِنْسَانًا فَهُوَ حَيَوَانٌ، لَكِنَّهُ إِنْسَانٌ، فَهُوَ حَيَوَانٌ.As for the exceptive syllogism: if the conditional in it is connective, affirming the antecedent produces the consequent, as in “If this is a human, it is an animal; but it is a human, so it is an animal.”وَاسْتِثْنَاءُ نَقِيضِ التَّالِي يُنْتِجُ نَقِيضَ الْمُقَدَّمِ، كَقَوْلِنَا: إِنْ كَانَ هَذَا إِنْسَانًا فَهُوَ حَيَوَانٌ، لَكِنَّهُ لَيْسَ بِحَيَوَانٍ، فَلَا يَكُونُ إِنْسَانًا؛And affirming the contradictory of the consequent produces the contradictory of the antecedent, as in “If this is a human, it is an animal; but it is not an animal, so it is not a human.”وَإِنْ كَانَتْ مُنْفَصِلَةً فَاسْتِثْنَاءُ عَيْنِ أَحَدِ الْجُزْءَيْنِ يُنْتِجُ نَقِيضَ التَّالِي، كَقَوْلِنَا: الْعَدَدُ إِمَّا زَوْجٌ أَوْ فَرْدٌ، لَكِنَّهُ زَوْجٌ، يُنْتِجُ: أَنَّهُ لَيْسَ بِفَرْدٍ، أَوْ لَكِنَّهُ فَرْدٌ، يُنْتِجُ: أَنَّهُ لَيْسَ بِزَوْجٍ، وَاسْتِثْنَاءُ نَقِيضِ أَحَدِهِمَا يُنْتِجُ عَيْنَ الثَّانِي.If it is disjunctive, affirming one of the two parts produces the contradictory of the other, as in “A number is either even or odd; but it is even”, which produces “It is not odd”; or “but it is odd”, which produces “It is not even.” And affirming the contradictory of one of them produces the other.

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