People mix these up all the time. Sometimes for malicious reasons (trying to “game” the system”) and sometimes just because it’s easier to look at the letter of the law. I am not referring to just legalisms, but rather instructions from teachers or bosses. The spirit of the more important aspect of the mission, but its often ignored because it takes too much time to interpret.
Ethan (10 year old) had to do a science experiment. His teacher said, “Make sure you do it all on your own.” So the letter of the law is clear, but what is the spirit of the law. The teacher clearly doesn’t want an experiment done by parents while the student watches TV. Parents often don’t know when to stop. They mean well, but can often go overboard. However, the spirit of the law is that your parents can’t help guide you. It’s just that they can’t dominate the exercise. Ethan got very upset when I was helping him a little because it went against the letter of the law.
At work, a boss may say, “I want this thing to launch by June 30.” What does he/she mean? It’s not that they are saying, “I don’t care about quality or features or anything excuses, launch whatever embarrassing thing you have on June 30!” The spirit of the message is, “We are making a target and are going to track towards that target. We must constant balance quality, deadline and scope to make sure the business has the best outcome.” It’s amazing to me how often people mix up the two messages.
People often muddle the two without even thinking about it. Part of UX design is thinking about the spirit of the product/service, and not just the list of features required. What are you trying to achieve at a more abstract level. How do you want your audience to feel? What is the larger point? What is the spirit of the law? These are the questions that lead to much better user experience.