The theory is that, like holes in Swiss cheese slices, all systems have deficiencies or inadequate defences. The causal trajectory of an incident leads to an incident when those deficiencies or failed defences in the system line up. It follows, then, that proactively increasing the defence layers reduces the likelihood of an incident. It also follows that in attempting to analyse an incident, a better understanding of that trajectory will uncover the absent or failed defences which enabled the system failure. Logically, then, in the aftermath of an incident, those system deficiencies are identified and addressed through corrective actions, thus reducing the holes on the Swiss cheese slices and therefore reducing the likelihood of a recurrence of the incident. But complex systems' incident trajectories are often unique. That is, addressing what went wrong in a particular incident will only help prevent that exact sequence from recurring. But the likelihood of the planets, or Swiss Cheese slices, aligning in exactly the same way is very remote. It is more likely that the next incident will involve a different trajectory and different holes on the Swiss cheese slices. Addressing the specific sequence that caused the incident will not address potential paths that the incident trajectory could have taken but for certain events. We often look at an incident sequence amazed, but relieved, that things were not much worse and that they could, in fact, have been, had it not been for some "lucky" event. But other than such a casual observation or a remark in an incident investigation report, little is done about those "other" non-causal events. That is, events that either prevented the incident from being of greater impact or the possible trajectory that did not occur -the road not travelled but which could have been travelled.
"most of the root causes of serious accidents in complex technologies are present within the system long before an obvious accident sequence can be identified". 2 F iii
That is, the holes are there, if only our investigation techniques could uncover all of them and not just those involved in the incident. Yet most investigation techniques are linear in their approach, seeking out the exact causal sequence - the truth of what happened - and then uncovering the root cause(s) which lead to that factual sequence.
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