Agreed that thought experiments are valuable tools that enable us to analyze principles without the distracting details of the real world. We can ignore questions about the relative worth of human lives, probabilities, and individual preferences. Therefore I conclude that, in the thought experiment, it is better to divert the train because forty is greater than five, and my reservations about altering the course of events I am not responsible for are not as important as keeping as many people alive as possible.

The conclusions we draw in the thought experiment apply to the real world to the extent that there are no specific, relevant circumstances that contradict the simplifying assumptions we made in the thought experiment. If we find out that the forty workers are in fact automatons, or they are men who have committed capital offenses, or they are comatose centenarians, or they are fertilized eggs, we may reconsider our conclusions.

I maintain that there is a specific, relevant circumstance that applies to every real-world scenario to which this thought experiment might be illuminating. In posing the problem, you guarantee that "The death toll is bound to be smaller" if Greg diverts the train. This is a certainty that we will never have in the real world, and this ambiguity will inevitably affect the way we think about the decision.