In a comment below, Jeff provides one version of an argument several people have made against what I've written about lines not existing:
Saying "treat people humanely" is one thing. "Saying torture is wrong," is another. They are related, but not identical. And for the latter, you simply must confront definitions. Sorry, but that means lines.
Well, no, "definitions" does not mean "lines," not in the sense I use it when I write, "There are no bright lines. There are no dim lines. There are no lines."
The lines the existence of which there is none are lines that divide a continuum into "moral" and "not moral" regions. (To this point, I haven't been intending to claim that there is no continuum for which such a line exists -- I've been thinking in terms of periods of stress for purposes of interrogation -- but it may be true in general.)
A definition doesn't draw a line on a continuum, it specifies membership in a set. The model from my previous post that goes with a definition is the disjoint sets, the red and green circles.
Now, if you draw a line on a continuum (you're really marking a point, but everyone calls it "drawing a line"), and you say, "Everything on this side of the line is permitted, everything on that side is prohibited," then what you've done is converted the continuum into the disjoint sets "permitted" and "prohibited." But you've done it as a matter of law, not as a matter of morality.
People say, "Soldiers need bright lines," and what they mean is, "Soldiers need clear rules." That's certainly true. But they also need (and are owed) morally good laws.
You can get goodness by "drawing a line" for those rules for which goodness can be, so to speak, good-enoughness. Speed limits divide speed into "legal" and "illegal," not into "safe" and "unsafe."
A speed limit can fail to be ideal in two ways. Driving at a certain speed may be legal but unsafe, or it may be illegal but safe. There are generally laws against reckless driving and such that can be used to penalize or discourage the first case. The second case -- well, nobody's too broken up about that imperfection; many or most drivers routinely drive over the speed limit, and for the most part cops don't enforce it very strictly. But no one denies that driving over the speed limit is against the law, and there's no pressing urge to change the law in order to minimize "illegal but safe" situations.
There is, however, a pressing urge to change the law in order to minimize the "illegal but moral" situations that might arise in interrogating terrorists. If you're thinking in terms of continuums and bright lines, though, "illegal but moral" and "legal but immoral" are directly linked. I'm not sure how you minimize the former without increasing the latter.