The trolley problem is a classic thought experiment in ethics. It goes like this:
A trolley is hurtling down a track towards five people. You are standing next to a lever that can divert the trolley onto a side track, saving the five people. However, there is one person on the side track. Do you pull the lever?
An alternative version goes like this;
A young, healthy person walks into a hospital. Five patients are in need of organ transplants. Do you kill the young person and harvest their organs to save the five patients?
This thought experiment is used to explore the ethical dilemma of whether it is better to act in a way that saves the most lives (utilitarianism) or to act in a way that respects individual rights (deontology).
The correct answer appears to be to pull the lever (or transplant the organs), saving the five people at the expense of the one person, maximizing the number of lives saved. However, there exist many arguments against this conclusion. Each as nonsensical as the last.
What is often ignored in this thought experiment is (un)certainty! In this case, we are certain that pulling the lever (or transplanting) will save the five people. However, in the real world, we are always uncertain about the consequences of our actions.
Some will argue that we can work with risk. We can evaluate the probability of different outcomes and act in a way that maximizes the expected value. However, this is also not possible, for a similar reason. We can never know the true probability of different outcomes given our actions.
Because we cannot know the future with certainty we can never evaluate one action as better than another.