In ethics, the plank of Carneades is a thought experiment first proposed by the Academic Skeptic philosopher Carneades of Cyrene. It explores the concept of self-defense in relation to murder.

In the thought experiment, there are two shipwrecked sailors, A and B. They both see a plank that can only support one of them and both of them swim towards it. Sailor A gets to the plank first. Sailor B, who is going to drown, pushes A off and away from the plank and, thus, proximately, causes A to drown. Sailor B gets on the plank and is later saved by a rescue team. The thought experiment poses the question of whether Sailor B can be tried for murder because if B had to kill A in order to live, then it would arguably be in self-defense.

The Case of the Speluncean Explorers by legal philosopher Lon Fuller is a similar exploration of morality and legality in extremis.

The title is also derived from a Greek legend originating from 2 B.C. The story was about two men who escaped from a sinking ship, one of them grabbed on a piece of plank which happened to be drifting along. However, when the second man tried to grab onto the plank to survive, the first man who was clutching the plank pushed him away for fear of the plank sinking if it held the weight of two people. The man who survived was later held for criminal trial, but was ruled as innocent because it was an act of self defence. This act of self defense when in times of danger which could have a probability of costing your life as well as others is forgiven until this day, and is not ruled as murder.^{[citation needed]}