A reader named Ruth writes:
This seems an opportune place to ask about a small issue that has been bothering me. I have always believed that when one uses the word “whether”, at least two options are going to follow. [“...whether sane or deranged”] and that if only one question was being posed, the correct word would be “if” [...“if sane”].
I encounter what I consider to be misuse of these words so regularly that I have begun to wonder how I got this notion in the first place. I would love to read some opinions (or categorical avowals) on the subject.
This is an interesting question, because “whether” certainly does imply the question “Which of the two?” Samuel Johnson describes “whether” as “a particle expressing one part of a disjunctive question in opposition to the other: answered by or.”
However, it is often used, and the Editor thinks quite correctly, with only one side of the question stated:
A Discourse of Things Above Reason: Inquiring Whether a Philosopher Should Admit There Are Any Such. (The title of a treatise published in 1681.)
In this case, the alternative “or not” is implied. There are still two alternatives, but only one needs to be stated. We could multiply examples, but one more will suffice:
The question then is, whether a Roman Catholic priest shall be compelled to disclose what he has received in confession, in violation of his conscience, of his clerical engagements, and of the canons of his church, and with a certainty of being stripped of his sacred functions and cut off from religious communion and social intercourse with the denomination to which he belongs.
Again, the implied alternative is “or not.”
So this much simpler sentence is perfectly correct:
I don’t know whether I’ll be there tonight.
And so is this one, which means the same thing:
I don’t know if I’ll be there tonight.
In some older references, whether is preferred in that situation.
Whether and if are not always interchangeable. Note the difference in meaning when two alternatives are expressed:
I don’t know whether I’ll be there tonight or tomorrow.
This means that I’ll be there either tonight or tomorrow—I don’t know which one yet.
I don’t know if I’ll be there tonight or tomorrow.
This means that I may be there tonight or tomorrow, or I may not be there at all.
To go back to the reader’s example, then, these are both correct:
I don’t know whether Letitia is sane.
I don’t know if Letitia is sane.
Note that, like its cousin either, whether can refer to more than two things:
I don’t know whether to pick Tom, Dick, or Hasdrubal.
Either and whether are always slightly uncomfortable in that role, because etymologically they imply a choice of two. But English does not provide us with a suitable particle for choices of three or more.