Before you begin a thing remind yourself that difficulties and delays quite impossible to foresee are ahead. You can only see one thing clearly, and that is your goal. Form a mental vision of that and cling to it through thick and thin.
— Kathleen Norris
And when they are impossible to see we need margin and reserve to protect our project from them. This margin is for the irreducible risks and the reserve is for buying down the reducible risks. Both irreducible (aleatory) and reducible (epistemic) uncertainties can be modeled for all projects. This is the role of risk management and project. To NOT model these uncertainties is to ignore them. To ignore them is to say to those providing the money that you're not following Tim Lister's advice...
Risk Management is how Adults manage projects
As well when you begin a thing remember another important quote...
Measurement is the first step that leads to control and eventually to improvement. If you can’t measure something, you can’t understand it. If you can’t understand it, you can’t control it. If you can’t control it, you can’t improve it. ― H. James Harrington
So if we're going to start a job and don't have an assessment - to some level of confidence - of how long it will take, how much it will cost, and what we will be capable of delivering when we get to the end of the money and the time - it is very likely that those paying for our work will be very disappointed in our efforts.
When we here that we can make decisions in the absence of knowing the probabilistic cost, schedule, and likelihood of producing the needed capabilities, think back to the two quotes above. And consider the conjectures below that requires us to ignore those quote and instead follow those statements in the absence of any evidence they are applicable outside of the Value at Risk being low enough that those providing the money don't really care if it's all a loss.
† Orginal post from Mark Anderson's email from ExecuNet, 9/14/2014