The Will cannot affect the nature of physical poisons.

But let them be asked, are they Teachers of Freewill or Changers of our Nature ? If a man eats by mistake from a deadly root, the Will of the eater cannot change that deadly thing, seeing that it is not an unfettered Will that he should change it; but it is an evil Constituent, the nature of which cannot be changed by words. How then can the just Judge condemn mankind (by asking), why they have not changed by the Will the evil Nature which cannot be changed by the Will ? [Ov. p. 48.] Therefore, let them either admit that unfettered free Wills are changed to Good or Evil or let them admit that if they are 'bound Natures' of Good and Evil, they are Natures which cannot be conquered by words. For they ought to supply an antidote as a medicine to counteract a deadly poison. For it is right that by natural illustrations that Teaching should be refuted which was composed deceitfully from analogous phenomena in Nature. But [xxi] Truth is strong enough to destroy with the single reply which it makes the numerous fabrications of Falsehood.