A lifeguard is on duty when a swimmer drowns. Arrange the events in the order the court reconstructs them.

1. The lifeguard is employed and takes up the post at the beach
2. A swimmer gets into difficulty within sight of the post
3. The lifeguard sees the difficulty and remains seated
4. The swimmer drowns
5. The court asks whether entering the water would have prevented the death with a probability bordering on certainty

The first event is doing the legal work, and it happens before any of the drama. Taking up the post is what creates the specific duty; without it the same inaction by the same person on the same beach is at most a failure to render assistance. The last step is the peculiarity of omission: since nothing was done, causation has to be established by asking what would have happened if it had been.