Authors/Ockham/Summa Logicae/Book III-1/Chapter 29

Latin	English
Cap. 29. De uniformi ex propositionibus de impossibili	Chapter 29. On uniform syllogisms from propositions de impossibili (of impossibility).
Post praedicta de uniformi ex propositionibus de impossibili est dicendum. Et est sciendum quod si propositiones omnes sumantur in sensu compositionis, talis discursus non valet, quia argueretur per istam regulam falsam ‘praemissae sunt impossibiles, igitur conclusio est impossibilis’	After the above, we must speak of uniform syllogisms from propositions of impossibility. And it should be known that if all propositions are taken in the sense of composition, such a discourse is not valid, because it would be argued by this false rule ‘the premises are impossible, therefore the conclusion is impossible’.
Similiter, si omnes propositiones de impossibili sumantur in sensu ƿ divisionis, non valet talis discursus, quia nullus talis in prima figura potest regulari per dici de omni vel de nullo. Unde sic arguendo ‘omnis homo impossibiliter est asinus; omne rudibile impossibiliter est homo; igitur omne rudibile impossibiliter est asinus’, patet quod non valet nec regulatur per dici de omni, quia per minorem non denotatur subiectum maioris verificari de aliquo sed magis removeri. Et ita cum quaelibet de impossibili aequivaleat uni negativae, et ex omnibus negativis non contingit arguere, manifestum est quod ex propositionibus de impossibili non contingit arguere.	Similarly, if all propositions about the impossible are taken in the sense of division, such a discourse is not valid, because no such in the first figure can be governed by to be said of all or nothing. Hence, thus arguing ‘every man is not possibly a donkey; every tamable thing is not possibly a man; therefore every tamable thing is not possibly a donkey’, it is clear that it is not valid nor is it governed by to be said of all, because by the minor the subject of the major is not denoted to be verified about something, but rather to be removed. And so since any proposition of impossibility is equivalent to one negative, and it is not possible to argue from all negatives, it is clear that it is not possible to argue from propositions of impossibility.