and the rule, in order to include implied as well as expressed negatives, questions as well as statements, will run thus:
The word used depends upon the writer's or speaker's opinion as to the reasonableness of the doubt, no matter in whose mind it is said to exist or not to exist.
1. If there is nothing to show that the writer considers the doubt an unreasonable one, the word is always 'whether', which reminds us that there is a suppressed alternative:
I doubt whether this is true (or not).
Every one is at liberty to doubt whether...(or not).
To this part of the rule there is no exception.
2. If it is evident that the writer disapproves of the doubt, the words introducing it amount to an affirmation on his part that the thing doubted is undoubtedly true; the alternative is no longer offered; 'that' is therefore the word:
I do not doubt that (i.e., I am sure that)...
Who can doubt that...?
This, however, is modified by 3.
3. The 'vivid' use of 'whether'. When the writer's point is rather the extravagance of the doubt than the truth of the thing doubted, 'whether' is often retained:
It is as if a man should doubt whether he has a head on his shoulders.
Can we imagine any man seriously doubting whether...?
Here, according to 2., we ought to have 'that', since the writer evidently regards the doubt as absurd. But in the first sentence it is necessary for the force of the illustration that the deplorable condition of the doubter's mind should be vividly portrayed: accordingly, he is represented to us as actually handling the two alternatives. Similarly, in the second, we are invited to picture to ourselves, if we can, a hesitation so ludicrous in the writer's opinion. We shall illustrate this point further by a couple of sentences in which again the state of mind of the doubter, not the truth of the thing doubted, is clearly the point, but in which 'that' has been improperly substituted for the vivid 'whether':
