A few quick thoughts in reply to Sasha's interesting response. Sasha writes:

As an initial matter, consider that, under many constitutional theories — which many people subscribe to — the Constitution "means" something independently of whether courts have recognized it. People holding such theories can differ on what that meaning is, and on what interpretive theory you should use to figure out what that is — for instance original public meaning or something else — but these people would each agree that there exists some meaning of the Constitution."

  It is true that groups of people can get together and agree on what the Constitution means, and for them that will become what the Constitution "is." For example, a group could decide that the Constitution has 14 branches of government overseen by a Giant Iguana King named Hector. They would be quite distressed that the courts have not yet recognized Hector the Iguana King as the authority ("damn activist judges!"). But to them Hectorism is not a theory of what the law should be but what it truly is.

  But as Sasha suggests, this is just a definitional point about what it means for the law to be something. To borrow from our former President, it depends on what the meaning of "is" is. "What Law Is" has filled libraries, and obviously I can't resolve it here. But I think I can take it to mean what I think most of us commonly think it means; what legal institutions believe it they must do or not do using widely shared practices of interpretation, rather than a person's view about what those legal institutions should believe.

  The contrary view makes the notion of what the law "is" rather useless, I think, which presumably explains why it is not the common use of the term. If the Constitution "is" what a person thinks it is, and no two people agree on what that is, then the Constitution is different to everyone. There isn't much to be gained by talking about what it "is" at that point. So we could define it that way, but the results are silly enough that it's not the common understanding of the term. And besides, Hector insists, and Hector must be obeyed.

  Sasha coninues:

But it may be misleading to claim that "Supreme Court doctrine" is the same thing as "constitutional law." For instance, it's also a view held by many that everyone who takes the constitutional oath — say all public officials — also have an independent duty to follow the Constitution. And there's no necessary reason why the Supreme Court's pronouncements on what the Constitution means should be authoritative. They're just one interpreter (well, nine) among many possible ones.

  This is another classic and much-debated question in the law reviews, of course, and again I can't settle it here. But I intentionally tried to sidestep that debate in my post by not referring only to the Supreme Court or "Supreme Court doctrine." Rather, I made a point to refer to "legal institutions such as courts." To elaborate on that, I think Congress is a legal institution. The executive branch is one, as well. Exactly what their role should be in determining constitutional meaning raises a fascinating set of questions, but those questions are distinct from the point I was making in my post.