When we think of safety it is usually by reference to its opposite, the absence of safety. The traditional view of safety, called Safety-I, has consequently been defined by the absence of accidents and incidents, or as the ‘freedom from unacceptable risk.’ As a result, the focus of safety research and safety management has usually been on unsafe system operation rather than on safe operation. In contrast to the traditional view, resilience engineering maintains that ‘things go wrong’ and ‘things go right’ for the same basic reasons. This corresponds to a view of safety, called Safety-II, which defines safety as the ability to succeed under varying conditions. The understanding of everyday functioning is therefore a necessary prerequisite for the understanding of the safety performance of an organisation.

(Under development, but you may also want to check the book.)