 What then are legal obligations? They are legal requirements with which law’s subjects are bound to conform. An obligatory act or omission is something the law renders non-optional. Since people plainly can violate their legal obligations, “non-optional” does not mean that they are physically compelled to perform, nor even that law leaves them without any eligible alternative. On the contrary, people often calculate whether or not to perform their legal duties. Could it be then that obligations are simply weighty reasons to perform, even if sometimes neglected or outweighed? This cannot be a sufficient condition: high courts have important reasons not to reverse themselves too frequently, but no legal obligation to refrain. Nor is it necessary: one has an obligation, but only a trivial reason, not to tread on someone’s lawn without his consent.