Authors/Ockham/Summa Logicae/Book II/Chapter 25

Latin	English
[2.25 DE CONVERSIONE PROPOSITIONUM DE POSSIBILI]	Chapter 25. On the Conversion of Propositions of Possibility
Circa conversionem propositionum de possibili est primo sciendum quod in isto capitulo accipiendum est semper 'possibile' secundum quod est cammune ad necessarium et ad contingens quod non est necessarium: ut 'possibile' sit idem quod 'propositia non impossibilis'. Et sic accipiendo 'possibile' concedendum est quod Deus potest esse Deus et quod homo potest esse animal.	Regarding the conversion of propositions of possibility, it must first be known that in this chapter we must always take 'possible' as it is a step towards the necessary and the contingent which is not necessary: ​​so that 'possible' is the same as 'a non-impossible proposition'. And in so taking 'possible' we must concede that God can be God and that man can be an animal.
Sic autem accipiendo 'possibile' est sciendum quod eaedem regulae quae dictae sunt de conversione propositionum de necessario' attendendae sunt circa conversionem propositionum de possibili. Nam illa de possibili, sumpta in sensu compositionis, et aequivalens ei est eodem modo convertenda sicut sua de inesse, quia in omnibus talibus conversionibus arguitur per istam regulam 'si unum convertibilium est possibile, reliquum est possibile', vel per istam 'si antecedens est possibile, consequens est possibile', quae regulae sunt semper verae.	But in taking 'possible' in this way, it should be known that the same rules that were said about the conversion of propositions of necessity' must be observed about the conversion of propositions of possibility. For the proposition of possibility, taken in the sense of composition, and equivalent to it, must be converted in the same way as its inherence, because in all such conversions it is argued by this rule 'if one of the convertibles is possible, the rest is possible', or by this rule 'if the antecedent is possible, the consequent is possible', which rules are always true.
Sed si sumantur tales propositiones in sensu divisionis vel eis aequivalentes, tunc non eodem modo convertuntur. Unde non sequitur 'Deus potest esse non creans, igitur non creans potest esse Deus', et hoc si subiectum consequentis accipiatur pro eo quod est. Similiter non sequitur 'uiator potest esse damnatus, igitur damnatus potest esse viator', sed sequitur 'igitur aliquis, qui potest esse damnatus, est viator'.	But if such propositions are taken in the sense of division or equivalent to them, then they are not converted in the same way. Hence it does not follow that 'God can be non-creating, therefore a non-creating thing can be God', and this if the subject of the consequent is taken for what it is. Similarly, it does not follow that 'a traveler can be damned, therefore a damned person can be a traveler', but it follows that 'therefore someone who can be damned is a traveler'.
Verumtamen ut plenius pateat qualiter tales propositones de possibili debent converti, est sciendum quod quanda subiectum propositianis de possibili est terminus communis vel includens terminum comniunem, propositio est distinguenda, eo quod subiectum potest supponere pro his quae sunt vel pro his quae possunt esse; quae regula intelligenda est, proportianaliter, sicut dictum est prius de propositionibus de praeterito et de futura. Si autem accipiatur subiectum antecedentis pro his quae sunt, tunc convertitur in illam de possibili, subiecto consequentis accepto pro his quae possunt esse, sed non accepto subiecto consequentis pro his quae sunt.	However, in order to make it more fully clear how such propositions of possibility should be converted, it should be known that when the subject of a proposition of possibility is a common term or includes a common term, the proposition is to be distinguished, because the subject can supposit for things that are or for things that can be; which rule is to be understood proportionally, as was said before about propositions about the past and the future. But if the subject of the antecedent is taken for things that are, then it is converted into that of possibility, with the subject of the consequent taken for things that can be, but not with the subject of the consequent taken for things that are.
Unde bene sequitur 'aliquis, qui est viator, potest esse damnatus, igitur aliquis, qui potest esse damnatus, potest esse viator'. Similiter sequitur 'Deus potest esse non creans, igitur qui potest esse non creans, potest esse Deus'. [Et non solum sequitur illa de possibili, sed etiam cum tali additione 'qui potest esse' sequitur illa de inesse, sicut sequitur 'Deus potest esse non creans, igitur qui potest esse non creans, est Deus']. Sed si subiectum antecedentis accipiatur pro his quae possunt esse, tunc sequitur illa de possibili, subiecto consequentis accepto pro his quae possunt esse, et non sequitur illa de inesse. Si autem praedicatum antecedentis sit terminus singularis vel pronomen demonstrativum sine addito, sequitur illa de possibili absolute; sicut sequitur 'creans potest esse Deus, igitur Deus potest esse creans', sive subiectum antecedentis accipiatur pro his quae sunt sive pro his quae possunt esse.	Hence it follows well that 'someone who is a traveler can be damned, therefore someone who can be damned can be a traveler'. Similarly, it follows that 'God can be non-creating, therefore he who can be non-creating can be God'. [And not only does that follow of possibility, but also with such an addition 'who can be' it follows of inherence, as it follows 'God can be non-creating, therefore one who can be non-creating is God']. But if the subject of the antecedent is taken for things that can be, then that proposition follows of possibility, with the subject of the consequent taken for things that can be, and that proposition does not follow of inherence. But if the predicate of the antecedent is a singular term or a demonstrative pronoun without an addition, that follows of possibility absolutely; as it follows 'a creator can be God, therefore God can be a creator', whether the subject of the antecedent is taken for things that are or for things that can be.
Per praedicta patet quod tales consequentiae non valent ‘videns potest esse caecus, igitur caecus potest esse videns'; 'verum potest esse impossibile, igitur impossibile potest esse verum'; 'nullum necessarium potest esse falsum, igitur nullum falsum potest esse necessarium'. Et hoc si subiectum Consequentis accipiatur pro his quae sunt; aliter consequentiae sunt bonae.	From the above it is clear that such consequences are not valid as ‘a seeing person can be blind, therefore a blind person can be seeing’; ‘a true thing can be impossible, therefore an impossible thing can be true’; ‘nothing necessary can be false, therefore no false thing can be necessary’. And this is if the subject of the consequent is taken for those things which are; otherwise the consequences are good.