S-D
I was looking at it from the viewpoint of assessing risks and developing risk mitigation plans, while being allowed to proceed with the activity. If the probability of occurrence is 100% (a certainty) and the impact or outcome is not what you wish, then one is not permitted to proceed until you've reduced the certainty to "only" a risk. Perhaps not a great example in this case, but whenever initiating an upgrade programme the probability (after about 1992) of not having a valid safety case was 100% (as it was not policy), so one had to first let a risk reduction contract to create one or resurrect the lapsed one, before being allowed to commit more money. That's what I meant by certainty. You don't proceed knowing it exists. Apply, for example, to Nimrod MRA4. There was a "no go" situation, mandated by Secy of State, 16 years before the fleet was scrapped.