You are comparing the sexiest problems of one field with the ordinary problems of another. This is just unfair comparison. By saying that "we know it when we s

You are comparing the sexiest problems of one field with the ordinary problems of another. This is just unfair comparison. By saying that "we know it when we see it", you are acknowledging that this is a value judgment (nothing wrong with that, just don't present it as an absolute truth). Also, by your definition, sending humans to the moon is not a hard engineering problem. It has been done before; it involves pasting together some relatively well-understood components like rocket engines and guidance systems.