Full Idea
In a shipwreck, suppose I have got hold of a plank which cannot carry two, if someone swims up with the intention of getting on the plank with me, thus destroying both of us together, I may use any amount of force to keep him away from the plank.
Gist of Idea
In a shipwreck I am entitled to defend my one-person plank by force
Source
Samuel Pufendorf (On the Duty of Man and Citizen [1673], I.05)
A Reaction
The limits of altruism. Presumably the force could include killing him. This example was often cited in the seventeenth century. The point is that Pufendorf is citing it as a self-evident principle of natural law. What if the other man is the King?
Book Reference
Pufendorf,Samuel: 'On the Duty of Man and Citizen' [CUP 1991], p.54