A couple of people asked in comments about my claim that, where a malleable desire comes into conflict with a fixed desire, that the malleable desire should give way. The claimed that there is nothing in the nature of either value that declares that the fixed desire has a natural right to rule in such a conflict, and the malleable desire has a natural duty to yield.
So, let’s assume that we are dealing with a run-away train. It is out of control. There is no way to stop it or to switch it to a different track. Ahead of the train there is a second train heading the opposite direction. This train, however, is under control. We have the option of moving this train onto a side track, so that the first train speeds harmlessly past.
As we are getting ready to switch the second train onto a side track, somebody chimes up, "Why should the second train yield the right of way to the first train? We have two trains heading towards each other who are about to collide.
There is absolutely no argument from nature – no intrinsic value – that declares that the first train has a natural right of way over the second train. Except for the fact that one train can be moved to a side rail and the other cannot, there is nothing that distinguishes these two trains – nothing that makes one train inherently better than the other.”
All of these claims about the intrinsic merits of the two trains are perfectly true. Yet, we can still ask what relevance these facts have to the question of whether or not to move the second train to a side track given the fact that moving the first train to a side track is not an option.
We are facing two option – moving the second train onto a side track, or a head-on collision. Moving the first train onto a side track and giving the second train the right-of-way is ex hypthesi not an option.
There is a principle in morality that says that 'ought' implies 'can'. It makes no sense to say that somebody 'ought' to do something that it is not possible to do.
It makes no sense to condemn a person for failing to teleport a child out of a burning building unless and until teleporting a child outside of a burning building is something he could have done. In that case, we can ask him why he did not do so and condemn him if his reasons-for-action are not good enough. Without that power, we can assume that even a person with the best reasons available would have failed to teleport the child out of the building.
Many people take this principle of 'ought' implies 'can’ to mean that morality requires some sort of counter-causal free will. However, I don’t see it that way. I see the principle of "ought" implies "can" to mean that we are going to focus social forces where they can have an effect, and ignore cases where they cannot have an effect. It is the moral equivalent of saying, "If you have two trains that are going to collide, one of which cannot be steered, you avoid the conclusion by steering the train you can."
If we were facing a different situation – if the first train could be switched to a side track, and the second train were this out-of-control train whose speed and direction we could not influence – then prudence suggests switching the first train, not the second.