epistemic or deontic?

Question

Which paraphrase would you use here?You must be mad to do that.1. I believe you are mad to do that.2. It is necessary for you to be mad in order to do that.

AlpheccaStars · Accepted Answer

#1 is a good paraphrase of the idiomatic expression.

Yankee · Answer

I'd expect "have to" (or an informal "gotta") rather than "must" to be used for the paraphrase in #2.

Anonymous · Answer

YankeeI'd expect "have to" (or an informal "gotta") rather than "must" to be used for the paraphrase in #2.

I see. So,in your variety, is have to used more often than must for objective obligation?

Thomas Tompion · Answer

It could mean either, Anon. It can be epistemic or deontic.

I think I'd expand 1. to "the fact that you do that means that you are mad'

Anonymous · Answer

2. It is morally binding for you to be mad to do that. Does not make much sense.

Thomas Tompion · Answer

No; 2 means that you can only do that if you are mad. 1. follows from it: the fact that you do it has caused me to believe you are mad.

I was wrong; they are both epistemic and logically linked.

Here's the syllogism:

Only mad people do X. (2)
You do X,
therefore you must be mad (1)

Anonymous · Answer

To paraphrase #2:

2. It is necessary for you to be mad in order to do that.

If you want to do that, you will first have to be mad.

Kooyeen · Answer

AnonymousYou must be mad to do that.

1. I believe you are mad to do that.

2. It is necessary for you to be mad in order to do that.

There's no difference.
#1 I believe you are mad to do that = (I believe) If you do that, you are mad.
#2 It is necessary to be mad in order to do that = If you are not mad, you don't do do that = If you do that, you are mad.

The only difference is that "I believe" that introduce #1. If that is the difference you want to point out, then #2 sounds like a kind of scientific truth compared to #1, with little room for subjective judgment.

Thomas Tompion · Answer

Kooyeen
AnonymousYou must be mad to do that.

1. I believe you are mad to do that.

2. It is necessary for you to be mad in order to do that.
There's no difference.
#1 I believe you are mad to do that = (I believe) If you do that, you are mad.
#2 It is necessary to be mad in order to do that = If you are not mad, you don't do do that = If you do that, you are mad.

The only difference is that "I believe" that introduce #1. If that is the difference you want to point out, then #2 sounds like a kind of scientific truth compared to #1, with little room for subjective judgment.

Hello Kooyeen,

I don't think they are actually synonymous, though the one might follow from the other. Look at it this way:

You hear that a child is in the habit of riding his bicyle quickly but blind, straight across a narrow road down which cars drive quite fast.
You say to him 'I believe you are mad to do that' - you acknowledge that the child does it and you think it a mad thing to do. You wouldn't say 'it is necessary for you to be mad in order to do that' partly because the in order to suggests that the act of madness hasn't yet been committed. You'd use formula 2 if you heard that the child was considering doing it for a bet, for instance, and were trying to dissuade him. In this second case formula 1, with its suggestion that the act is frequently being committed would become inappropriate: after all it's not I believe you would be mad to do that.

Anonymous · Answer

More precise for me:

You must be javascript:void(0) to do that.

1. I believe you are mad to do that.

2. It is required that you be mad if you want to do that.

I think the, revised, second paraphrase is not a suitable one for "You must be mad to do that". The "must" in that statement is "epistemic" and I've not met anyone who feels it could also be used deontically there. "Have to be" would suit the need to express deontic modality there.

epistemic or deontic?

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