We see things in the world that can exist and can also not exist (sunt possibilia esse et non esse)—I mean those that can be generated and can be destroyed. Now everything that can exist [and can also not exist] has a cause. For since on its own it is related indifferently to those two—existing and not existing—if existing is its status (e/ approprietur), that must be on the basis of some cause. But one cannot go on ad infinitum in [a series of] causes, as was proved above on the basis of Aristotle's reasoning. Therefore, one must posit something the existing of which is necessary (aiiquid quod sit necesse
14 The editors of the Marietti edn. of SCG suggest that this Latin expression stems from Avicenna's Arabic, and they distinguish it typographically (although not in all its occurrences). See their note to this passage. But they offer no evidence that Aquinas derives the expression from the medieval Latin translation of Avicenna, and I see no reason why he should have had to do so.
Now everything necessary either has the cause of its necessity in something else, or it doesn't but is, instead, necessary through itself. But one cannot go on ad infinitum in [a series of] necessary beings that have the cause of their necessity in something else. Therefore, one must posit some first necessary being that is necessary through itself. (15.124)
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