Assumptions: must, can and should ?

Question

When making assumptions, aside from 'must', could 'can' and 'should' be used too? I've thought of the following examples:

1) James must/should be in India by now.

2) This can't be the man that's won the lottery.

CalifJim · Accepted Answer

Those modals can all be used as you say, yes.

James must be in India by now says The only logical conclusion is that James is now in India.
James should be in India by now says It is [to be expected / quite probable] that James is now in India.

This can't be the man that's won the lottery says The only logical conclusion is that this is not the man who has won the lottery. (It must be that he is not that man.)

CJ

EyeSeeYou · Answer

I see. And what about the opposite forms for each of those modals? Would you say they still express probability and assumptions?

James must not be in India by now.

James should not be in India by now.

This can be the man that's won the lottery.

CalifJim · Answer

Unfortunately, no. Modals are very sensitive to negation. The way you used them first is how they are normally used. The 'opposite' of must is cannot. And should is a weaker form of must.

On occasion, especially in AmE, you'll hear must not with the same meaning as cannot.
Sometimes you hear the negation of should in the sort of meaning you're asking about.

For logical deductions based on known facts, these are the usual modal formulas.

The light is on; he must be home.
The light is on; he [may/might/could] be home. Not: He can be home.

The light is off; he can't be home. [= he must not be home (much less often)]
The light is off; he [may / might] not be home. Not: He [can/could] not be home.

OK: It's already 6 o'clock; he should be home.
Not OK: It's not yet 6 o'clock; he shouldn't be home.

See

CJ

Kooyeen · Answer

CalifJim Not OK: It's not yet 6 o'clock; he shouldn't be home.

Why not?

I always thought "shouldn't" was the opposite of "should". I use it often that way, and that shouldn't be a problem, right?

Loojka · Answer

CalifJim Modals are very sensitive to negation. The way you used them first is how they are normally used. The 'opposite' of must is cannot.

Yes, and 'needn't' can also be the opposite of 'must':
I must go home. ? I needn't go home yet.

'Mustn't' means 'not allowed to'.

CalifJim · Answer

that shouldn't be a problem, right?

Right. It shouldn't be, but it is.
shouldn't can be used that way, but you can't, in my opinion, state a blanket rule that makes it universally idiomatic in that meaning.

It does not work (to my ear) for It's not yet 6 o'clock; he shouldn't be home.
This one is too confusable with the shouldn't that means he is obliged not to be home, it would be wrong of him to be home at this time.

Here's why:

If the subject of the sentence is a person, the confusion and ambiguity is much more likely, because a person can be obliged not to do things.
If the subject is inanimate or some general abstract situation, it's much easier to sort out which meaning of should not is intended.
______________

Another complication: Sometimes the distinction between the progressive and the non-progressive forms makes a distinction between the shouldn't of logic (probability) and the shouldn't of obligation (advisability).

She shouldn't be arriving before 8. [It's not probable she'll arrive before 8.]
(She only started at 7:30 and it takes 45 minutes to get here.)

She shouldn't arrive before 8. [It is not advisable for her to arrive before 8.]
(It would make a bad impression on the hosts of the party. They won't be ready to receive guests yet at that time.)
______________

English modals: As soon as you think you understand them completely, things get more complicated.

CJ

CalifJim · Answer

Loojka,

The original poster was asking about the modals as used in logical deduction, not the modals as used in social interactions such as obligation and freedom from obligation. For this reason, I didn't want to confuse the issue by bringing in information about the second group.

But you are absolutely right -- within the world of social interaction -- i.e., within the 'second group' I mentioned above. There, the 'opposite' of must (or have to) is needn't (or don't have to). Either someone is under the constraint of some obligation, or one is free from that constraint.

CJ

Loojka · Answer

CalifJimLoojka,

The original poster was asking about the modals as used in logical deduction, not the modals as used in social interactions such as obligation and freedom from obligation. For this reason, I didn't want to confuse the issue by bringing in information about the second group.

I understand what you meant, just wanted to add another meaning. It was not my intention to confuse the issue either, that's exactly why I provided the example. You were talking about the negation of the modals, pointing out the possible misunderstanding that might occur when they are being used in negative form (that's where my students make mistakes very often), so I just thought that an additional example can be useful. No hard feelings

CalifJim · Answer

that's where my students make mistakes very often

Not at all surprising! It's a difficult topic to master!

CJ

Assumptions: must, can and should ?

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