logical conclusion Options

[navi]

He believes that the earth is flat.

1-Then he has to be crazy.
2-They he should be crazy.
3-Then he ought to be crazy.
4-Then he must be crazy.

Are all of the sentences 1 to 4 acceptable in response to the first sentence.

[Sparky]

No, sentences 1 and 4 are correct. The others don't make sense in this context.

[Tinker Grey]

Agreed.

[wonderwhy]

He believes that the earth is flat.

1-Then he has to be crazy.
2-They he should be crazy.
3-Then he ought to be crazy.
4-Then he must be crazy.

Are all of the sentences 1 to 4 acceptable in response to the first sentence.

I disagree [partially] with Sparky and Tinker. I'd say that numbers 1, 3 & 4 work here, though 'ought to' is not as likely as 'likely/probably'.

There's no reason that a person can't soften the conjecture; 'ought to' and 'should' equate epistemically to 'probably/likely'.

Then he's probably/likely crazy.

Then he may [well] be crazy.

Then he might [well] be crazy.

The reason 'should' can't be used is that epistemic 'should' isn't used in English for pure speculation. Epistemic 'should' is used when the party has a measure of personal knowledge regarding some issue/situation/etc.

'should' here feels ever so slightly that it has a deontic/advice meaning and a sentence like,

Then he should be slapped upside the head/Then he should be put in a mental ward would fit semantically.

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EDIT:

Having given it more thought, I have changed my mind. In this particular situation, 'ought to' seems to have as firm an epistemic sense as 'should'. I believe that Sparky and Tinker are right in this case.

If I'm not mistaken, this issue has come up before wherein 'ought to' did not match 'should' in an epistemic sense. That particular situation and other potential examples elude me now.