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What's the difference between must and should when used for logical deduction? Can they be used interchangeably? Here's an example:

"You must be tired after your long journey." - MUST

"I've revised so I should be ready for the test." - SHOULD

Is it correct to write "You SHOULD be tired after your long journey" ?

Is it correct to write "I've revised so I MUST be ready for the test" ?

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  • No, they're not interchangeable. You can write all four of those sentences, but the pairs have slightly different meanings.
    – Mitch
    Commented Apr 13, 2016 at 16:13

2 Answers 2

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While both must and should are "square" modals -- the ones that mean logical Necessary,
which is represented by   ⃤ in formulae -- their usage in English shows differences.

As deontic modals, must means 'obligatory', while should means 'strongly advised'.

  • You must include your phone number; otherwise it's incomplete.
  • You should include your phone number; we may need to contact you.

But as epistemic modals, should is not just a weak must -- it appears to refer to the variety of cognition involved in drawing the conclusion.

  • I see a lot of cars in the parking lot; they must be open.
  • It's after 9:30; they should be open.

In the first example above, must is used to express a conclusion from evidence or experience;
while in the second, should is used to express a conclusion from theory or expectation.
You could say epistemic should is a priori and epistemic must is a posteriori.

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  • Also, 'should' has (at least) two distinct senses, one a recommendation to do something, and another an expectation that something might happen (another way to describe your two cases.
    – Mitch
    Commented Apr 13, 2016 at 16:12
  • I don't agree that the should of the example is a strong modal operator. *"You must be tired, but you aren't necessarily tired" is contradictory. But "You should be tired, but you aren't necessarily tired" is not contradictory.
    – Greg Lee
    Commented Apr 13, 2016 at 16:34
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    Is there a sub stack where all the questions are asked and answered with such high caliber? Bravo, sincerely.
    – Jonathan Piccirilli
    Commented Apr 14, 2016 at 14:26
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Big thanks, that's the essence:

"Must is used to express a conclusion from evidence"

and

"Should is used to express a conclusion from theory"

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  • True only of the epistemic senses, not the deontic ones.
    – John Lawler
    Commented May 7, 2016 at 23:26
  • What do you mean by theory? Does it mean expectation ?
    – yubraj
    Commented Jun 4, 2016 at 11:52

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