Authors/Ockham/Summa Logicae/Book II/Chapter 33

Latin	English
[2.33 DE PROPOSITIONE DISIUNCTIVA]	33: On the Disjunctive Proposition
Disiunctiva est illa quae componitur ex pluribus categoricis mediante hac coniunctione 'vel', vel mediante aliquo aequivalente sibi. Sicut ista est disiunctiva 'tu es homo vel asinus'; similiter ista est disiunctiva 'tu es homo vel Sortes disputat'.	A disjunctive is a proposition that is composed of several categoricals with the mediating conjunction 'or' or with something equivalent to it. For example, this is a disjunctive proposition: 'You are a man or a donkey', likewise, this is a disjunctive proposition: 'You are a man or Socrates is disputing'.
Ad veritatem autem disiunctivae requiritur quod aliqua pars sit vera; et hoc est intelligendum quando propositiones sunt de praesenti et non de futuro nec aequivalentes propositionibus de futuro. Et hoc diceret Philosophus; tamen secundum veritatem ad veritatem disiunctivae requiritur quod altera pars sit vera, quia secundum veritatem propositio de futuro est vera vel falsa, quamvis evitabiliter.	Now for the truth of a disjunctive proposition it is required that one part be true, and this is to be understood when the propositions are present tense, and not future tense or equivalent to future tense propositions. And this is what the Philosopher would say, yet in fact for the truth of a disjunctive proposition it is required that either part be true, because in fact a future tense proposition is either true or false, though avoidably.
Sed ad necessitatem disiunctivae non requiritur necessitas alterius partis, sicut ad hoc quod haec sit necessaria 'Sortes sedet vel non sedet' non requiritur quod altera pars sit necessaria. Tamen ad necessitatem disiunctivae requiritur [vel] necessitas alicuius partis, vel quod partes sibi contradicant, vel aequivaleant contradictoriis, vel sint convertibilia cum contradictoriis. Unde haec est necessaria 'Sortes currit vel Deus est', quia altera pars est necessaria; haec autem est necessaria 'Deus creat vel non creat', quia partes contradicunt.	But for the necessity of a disjunctive proposition, neither part is required to be necessary. For example, in order for 'Socrates is sitting or not sitting' to be necessary, it is not required that either part be necessary. Nevertheless, for the necessity of a disjunctive proposition it is required that one or other part be necessary or that the parts contradict each other, or that they be equivalent to contradictories or that they be convertible with contradictories. Hence, this is necessary: 'Socrates is running or God exists'. for the second part is necessary, but 'God creates or does not create' is necessary because the parts contradict.
Ad possibilitatem autem disiunctivae sufficit quod altera pars sit possibilis; sed ad hoc quod distiunctiva sit impossibilis requiritur quod utraque pars sit impossibilis.	Now for the possibility of a disjunctive proposition it is sufficient that either of its parts be possible. But for a disjunctive proposition to be impossible it is required that both parts be impossible.
Sciendum est etiam quod opposita contradictorie disiunctivae est una copulativa composita ex contradictoriis partium ipsius disiunctivae, et ideo idem sufficit et requiritur ad veritatem oppositae disiunctivae quod sufficit et requiritur ad veritatem copulativae.	It should also be noted that the contradictory opposite of a disjunctive proposition is a copulative proposition composed of the contradictories of the parts of that disjunctive proposition, and therefore the same thing that is requisite and sufficient for the truth of the opposite of a disjunctive proposition is requisite and sufficient for the truth of a copulative proposition.
Sciendum est etiam quod ab altera parte disiunctivae ad totam disiunctivam est bonum argumentum, et e converso est fallacia consequentis, nisi sit aliquando aliqua causa specialis impediens fallaciam consequentis. Similiter a disiunctiva cum negatione alterius partis ad alteram partem est bonum argumentum, sicut bene sequitur 'Sortes est homo vel asinus; Sortes non est asinus; igitur Sortes est homo'.	It should also be noted that it is a good argument from either part of a disjunctive to the whole disjunctive proposition, but the converse is the fallacy of the consequent, except that sometimes there is some special cause blocking the fallacy of the consequent. Likewise, it is a good argument from a disjunctive proposition plus the negation of either of its parts to the other part. For example, this is a good consequence: 'Socrates is a man or a donkey; Socrates is not a donkey; therefore Socrates is a man'.