There was the following sentence in the article under the caption “Find your happiness “in iVillage. com.: http://www.ivillage.com/what-learn-annoyingly-happy-people/4-b-479266#ixzz2PiSjFoXw

“If there are two words that are keys to contentment, they are ‘limits’ and ‘boundaries,’” says Reiss. “Prioritizing the must-do items on our lists is essential, after which the should-do list must be narrowed down with polite but firm no-can-do responses.”

It seems the “must-do” things should be prioritized to “should-do” things in making decisions. Is “Must” an order you must obey, and “should” is a mere recommendation that you can always or easily neglect?

What is the basic difference of “must-do” from “should-do” in terms of the level of obligation, moral and motivational aspects?


A must-do is an unavoidable requirement; a should-do is no more than a desirable goal.

  • Very succinctly put. I was trying to think of something to add which might help the OP more, but this says it all. – Mynamite Apr 6 '13 at 21:17
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    That's one of the basic differences. There are actually two factors involved. First, this is for deontic modals only; epistemic modality works differently. Second, whereas must and its paraphrase have to are "strong obligation" modals, should and ought to are said to be "weak obligation". And one big source of a distinction between what you have to do and what you should do is outside versus inside motivation for the obligation. That's where the desire in a desirable goal comes from. Whereas obligation is almost always externally supplied. – John Lawler Apr 7 '13 at 0:17

The RFC 2119, specifies how must and should should be interpreted in other Request for Comments' (RFC), which are technical specifications.

1.MUST This word, or the terms "REQUIRED" or "SHALL", mean that the definition is an absolute requirement of the specification.

3.SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a
particular item, but the full implications must be understood and
carefully weighed before choosing a different course.

You can interpret must and should in the same way for non-technical documents too.


Must do implies necessity or inevitability and is a firm imperative or obligation, while should do implies subjective desire or an optimal outcome.

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