What makes a legal question "hard"?

When someone says another person has a brilliant legal mind, I assume that means the person is well suited for hard legal questions. I think mathematicians and logicians have a systematic way of determining how hard their questions are. Is there a similar way of determining how hard a legal question is?

From Computational Complexity Theory (Wikipedia):

NP-hardness (non-deterministic polynomial-time hard), in computational complexity theory, is a class of problems that are, informally, "at least as hard as the hardest problems in NP". More precisely, a problem H is NP-hard when every problem L in NP can be reduced in polynomial time to H.[1]:80 As a consequence, finding a polynomial algorithm to solve any NP-hard problem would give polynomial algorithms for all the problems in NP, which is unlikely as many of them are considered hard.[2]

A common mistake is thinking that the NP in "NP-hard" stands for "non-polynomial". Although it is widely suspected that there are no polynomial-time algorithms for NP-hard problems, this has never been proven. Moreover, the class NP also contains all problems which can be solved in polynomial time.

• As a mathematician, I can tell you that we really don't have a systematic way of determining how hard a question is. It's very subjective. I expect law is the same. Commented Nov 8, 2015 at 15:01
• I edited my question to show the example of "np-hardness" Commented Nov 8, 2015 at 18:58
• @Mr.A NP-hard is a technical term about the time it takes a computer to solve problems in a certain class; the problems are in fact often very easy to solve, just very slow. Trying to make that analogous to whether a legal problem is difficult is like drawing an analogy between an irrational law and the square root of 2. Commented Nov 8, 2015 at 19:52
• @NateEldredge it is very much the same. In fact, lawyers, judges, and legal scholars will have fervent debates about whether case X is "hard" probably about as often as debates over what the actual outcome of case X should be. Indeed, very often it's true that if you were able to know whether Justice John Smith thinks a case is "hard" or not that would greatly aid your assessment of which way Justice Smith will probably vote on the outcome of the case. (If that makes sense.) Commented Nov 9, 2015 at 8:42
• Justice Potter Stewart's obscenity definition sets the precedent: "I know it when I see it" Commented Nov 9, 2015 at 13:45