Principles are those opinions you have about what's right and smart to do in life, and what isn't. They're just opinions and everyone doesn't agree on them by a long shot, but they can still be a really useful, perhaps essential, part of living a good life…if they're used properly. If not, they're just words, words that waste your time now and make you look and feel like an a-hole later.
Principles include things like "Don't lend or borrow money," "Be faithful to your spouse," and "Don't make major decisions too quickly." They're usually, but not always, in this form — what's called a hypothetical imperative, which is a dictate based on a potential (hypothetical) situation. I'm not a fan of using hypothetical imperatives as the rules by which to live one's life (hence why none of my Higher Laws are of this type–more on hypothetical imperatives here and here), because they're simplifications and don't apply to everyone in every situation — but that doesn't make them useless. Laws are one thing; principles are another.
Principles give you guide-wires in specific situations. If I have a firm principle about spousal fidelity and I'm confronted with the opportunity to cheat, then I have a clear and automatic reason not to: I can, by way of my principles, pre-decide, outside the heat of a difficult moment, how I would like to handle it.
But of course principles only work if you, you know, use them. Something that doesn't seem to be well-understood (at least in people I'm confronting lately) is that a principle is developed in the off-hours, when there's nothing specific happening (at least to you) and time to think, and used in the thick of things, when you have to make a difficult decision. Some people seem to be all about the having of principles in-between tough decisions, but when the situation the principle is hypothesizing actually happens, they throw the principle away.
And this is easy to do, which is the main reason you have to be willing to try not to do it — it's super easy to tell yourself, "Well, when I formed that principle, I had no idea I'd wind up in this particular situation" — of course you didn't; that's the point. This situation is probably highly emotionally charged and tricky to think clearly in, and that's why you took the time beforehand to form a principle about what to do in these types of situations.
And that was a wise idea: There's a reason we value principles, principled people and principled actions — they make for better decisions, 99% of the time, over making snap judgments in the middle of heated situations.
Example 1: Two different men have one too many drinks and wind up in fistfights. Both win; both cock back for the final devastating blow. One hasn't really given situations like this much thought; the other has thought, said and maybe even written about how important it is to use only just as much violence as necessary, and no more. Those thoughts, and the cognitive dissonance created by being about to act contrary to so many prior statements, penetrate the adrenaline and the alcohol and stay Guy Two's hand. Guy One is now in jail for seriously injuring someone.
(A similar example that may speak to more people is having a principle about not driving while intoxicated…I personally have seen that principle stop, and not stop, numerous tragedies.)
Example 2: Two different women are passionately kissing men they'd really like to be in bed with, like, right-to-the-now. Neither has access to birth control at the moment, but Girl One has a die-hard principle of don't mess around when it comes to birth control to fall back on. She pulls back, and either engages in a lesser sex act for now, or learns in the nick of time that this guy is one of those douchebags who will try to push you into unsafe acts (and thus avoids a relationship that never should have been anyway). Girl Two is now staring down the barrel of a much, much worse situation.
(For what it's worth, I've been both Guy Two and Girl One; and I've known both Guy One and Girl Two. So those are not out-of-my-butt examples at all.)
So principles are great–different from Laws but still great to have–and can really improve the quality of your life. But getting any use out of them requires two separate works: First, carefully form them and Second, USE THEM when the time comes. Generally speaking, if you have to talk yourself out of one of your principles in order to do something, you'll probably regret it — and it's a good idea to make it as difficult as possible to talk yourself out of them, especially the ones you really believe in.
In the heat of a tricky situation, cognitive bias is in full swing, and your wider vision is impaired by emotional manipulations — yours and other people's. That's exactly when you want to have good principles, and the guts to follow through and use them. So why the tendency not to?
I suspect it comes down to Wanting To Be Right All The Time — we don't like admitting that there are such things as situations that make us lose control, where we can be manipulated by others and by our less-good selves. But this fear is a self-fulfilling prophecy: By letting it talk you out of using your principles when they're needed, they make you way more manipulable than you would be otherwise.
Know Thyself, know your weaknesses, and put measures in place to shore them up in times of need. (Oddly enough, this is advice I often give relating to polyphasic sleep, too. Any endeavor that requires going up against your own weaknesses benefits from pre-thinking and principles.)