The Data Against Kant: Ought vs Can
THE history of moral philosophy is a history of disagreement, but on one point there has been virtual unanimity: It would be absurd to suggest that we should do what we couldn’t possibly do.
.. In other words, we might indeed say that someone ought to do what she can’t — if we’re blaming her.
.. Even when we say that someone has no obligation to keep a promise (as with your friend whose car accidentally breaks down), it seems we’re saying it not because she’s unable to do it, but because we don’t want to unfairly blame her for not keeping it. Again, concerns about blame, not about ability, dictate how we understand obligation.