The negative formulation of the GR has at least instrumental importance:
|Parent:||Do not do evil.|
|Child:||What is evil?|
|Parent:||That’s an involved question, but let’s start with pain and the things and experiences that are feared.|
|Child:||What’s pain and what’s feared?|
|Parent:||Think about what causes you pain. This’ll give you an decent idea of the nature of pain as such, as well as of what things tend to cause pain. You are human, like your fellows. If you are punched in the nose, you’ll feel pain. Therefore, as part of “not doing evil,” do not punch other people in the nose.|
We share similar minds (e.g. logic) and bodies that work somewhat similarly. That’s a start.
But its positive formulation is outrageous. Every person is different with his own unique preferences. To look at oneself for clues as to what others want is narcissism extraordinaire.
For example, if the third assumption does not hold, and I do know how do promote Smith’s welfare, then the GR is entirely superfluous. I just do what’s good for Smith without undertaking any “imaginative role reversals.”