But it is a bad rule. But what is essential depends on the relationship between the precedent-case and the new case before us that demands a judgment, the case for which it will be a precedent. Determining what is essential is difficult enough. Now we see that the essential varies with different purposes. There is no single rule of a case.
Every case is the intersection of a thousand strands of historical meaning.
By itself it points in no particular direction. It becomes 'directional' only when we have a new case to decide and go back to the older case in order to explore its similarities and differences with our own case. As in geometry, it takes two points to make a line. It may well depend too on how we want our own case to be decided. Clearly, there is more art than rigorous science to legal reasoning.