The last three of the five items in the list of what safety factors should protect against all refer essentially to errors in our theory and in our application of it. They, therefore, are clear examples of uncertainties that are not easily amenable to probabilistic treatment. The eventuality of errors in our calculations or their underpinnings is an important reason for applying safety factors. This uncertainty is not easily reducible to probabilities that we can determine and introduce into our calculations. (It is difficult to see how a probability estimate could be accurately adjusted to compensate self-referentially for the possibility that it may itself be wrong.) It follows from this that safety factors are best understood in a framework of safety engineering that includes the reduction of both risk and uncertainty.