Souter v. Simpletons United
... the Constitution is no simple contract, not because it uses a certain amount of open-ended language, but because its language grants and guarantees many good things, and good things that compete with each other and can never all be realized, altogether, all at once.
-- Justice David Souter
On rare occasions, the Constant Weader is smart enough to let wiser fellows do the talking. In his Harvard commencement address, Justice David Souter explains why it isn't easy to interpret the Constitution. This is a tightly-packed analysis, though Souter makes it as understandable to the layperson as possible. If you want to know what the Third Branch of the federal government is supposed to do, this is as good an explanation as you're likely to hear.
BTW, I noticed some newswriters (like Boston Globe reporter Jonathan Saltzman) characterized the speech as one in which the Justice "defends judicial activism." Well, no, he doesn't. I should say, rather, than he does quite the opposite. Don't believe everything you read in the papers.
Harvard Commencement Speech, Parts 1 thru 3:
Here's the text of Justice Souter's remarks, as prepared.
Update: CW: if you haven't read enough of my marvelling at how smart David Souter turned out to be (admittedly, he benefited from low expectations), Linda Greenhouse, in recapping Justice Souter's Harvard commencement address does the same. I also recommend the reader comments, particularly Nos. 13 & 20.
Update 2: Dahlia Lithwick in Slate: "David Souter finally tells America to grow up."