With regard to the Lucretius quote (and with the hope that Don might help with translations), I think maybe we would be well advised (today) to replace "certainty" with "reliance." What can we -- must we -- rely upon? And that, I think, Epicurus nailed (and, again, something that I suspect Sextus just misunderstood)
LOL! You know I'm always up for a good translation exercise, even in Latin. To which Lucretius quote are we referring?