The most basic law of design, for the 20th century at least, has been form follows function. The idea that objects should be created based on the action they are used for. With the digital world today, the law is a bit broken. But I claim that the law still works, with just a slight tweak. I just saw Objectified, the fantastic documentary about industrial design. One of the designers calims that the original law of design form follows function doesn't apply anymore because design today has become more and more digital, more abstract. With objects like the iPhone, with all its functions, form cannot represent what it does. It is too complex.
But the law is still sound. If we abstract the purpose of the law a bit, it means that any product should really become its function, a pair of scissors is really nothing but the function for cutting. For two reasons this is a good thing: An object that objectifies its function is effective. An object that objectifies its function is simple to understand for the user. Scissors are rarely used inefficiently or misunderstood but it's users.
When we go digital, we remove the analogue function of the object. It can no longer have a shape based on that function because the function does not exist in the real world. Now, a certain element of the object will always be part of the real world, the interaction with the object.
And this is where the law comes into play again. If we think about the function of and object, not as a physical movement or action, but as an interface for a human being to perform a function, the interaction itself becomes the function of the object.
Some may argue that the abstract function of the object, e.g. gaming or texting on an iPhone, is the main function of an object. But that function also has an abstract layer of interface, the GUI, for that action. This is form and function for an abstract object or function.
So deconstructed, the law of design transformed for today world would read: Form follows interaction.