How do you make big, life-altering choices like whether to keep your corporate gig or pursue your entrepreneurial dreams, say, or whether to give up a successful but unsatisfying career path to try another you think would suit you more?

Most of us know that old stand-by, the pros-and-cons model. In this classic approach to decision making, you simply tote up the positives and negatives of your alternatives and, presto chango, you magically reach a thoroughly logical decision. Except if you've tried this, you know it has one glaring flaw -- your emotions often fail to heed your tidy list. One option might have a tally of positives as long as your arm and the other apparently nothing going for it, but your heart still longs for the seemingly unsound choice.

Other frameworks for making hard choices take this very human problem into account. Philosopher Ruth Chang, for instance, argues that, rather than thinking of these decisions as an exercise in logic, we celebrate them as opportunities to redefine our identity, choosing based on our aspiration for who we want to be rather than some (possibly non-existent) standard of what is objectively "best."

But for the toughest decisions of our lives we can use all the tools we can get. A new book on the subject called The Crossroads of Should and Must offers yet another alternativeIt's by  Elle Luna and it's based on a blog post that went viral back in 2014.

Is your should getting in the way of your must?

The original post is well worth a read if you didn't encounter it when it was published (I was totally late to this party too), but for an even more succinct introduction to Luna's thinking, check out a recent interview she did with VC Hunter Walk. In it, she talks about her own crossroads moment when, burnt out from working as a designer at a startup, she decided to step away without firm future plans and pursue her interest in art. How did she make a call that would have most of us wringing our hands for months or years? She divided her arguments into two buckets, "should" and "must."

"Should is how other people want us to live our lives. It's all of the expectations and obligations that others layer upon us. Sometimes shoulds are small, like 'You should check out this movie!' Other times, shoulds are large and insidious, like 'You should sit in the back of the bus because of your skin color,'" she tells Walk.

On the other hand, "must is who we are, what we believe, and what we do when we are alone with our truest, most authentic self. It's our instincts, our cravings and longings, the things and places and ideas we burn for, the intuition that swells up from somewhere deep inside of us," she explains in her original essay.

It's easy to talk about going with what you must do and forgetting what you should do, but getting to that point, Luna tells Walk, actually involves doing the hard mental work of "shedding shoulds." If we make previously unconscious "shoulds" conscious and examine whether they are right for us, "they loosen their grip on our lives," she contends. And it's only when we clear away this tangle of shoulds that we can follow the dictates of must.

"Should is the doorkeeper to must, the counterforce that works against even our very best intentions. So long as should has a grip on us, our must doesn't stand a chance!" she insists.

What's your must?

So what does that boil down to for those who need to make big, scary life decisions? Luna's wisdom suggests a simple action plan: Examine your reasons for pursuing each path and consider where they're coming from. Is this a should you picked up along the way from your parents or society at large? Or is this a must that's coming from somewhere way down deep in your being? Then opt for must.

Of course, it's not so simple (what about money? what about fear?), and Luna's book no doubt addresses those complications, but even the basics of her framework offer another useful tool for those standing at a crossroads. Forget the pros-and-cons list, and instead ask yourself, not what you should do, but what you must do.