Whenever you start a sentence with “what’s to say” or “who’s to say,” you need to complete the sentence with a full clause and a question mark. This is because these sentences are used for expressing the uncertainty of an event or a posited truth. Here are a few examples.
What’s to say he wanted to go to the party in the first place?
Who’s to say that we’re to blame for global warming?
Who’s to say what I can and cannot do?
Just like when it comes to ending sentences in a preposition and maintaining a consistent parallel structure, using “what’s to say” or “who’s to say” to start a sentence is largely a matter of personal preference. Some people like this kind of writing style, whereas others feel that it is too casual for certain applications.
If you separate the respective contractions, you can see how these sentences are still grammatically acceptable. “Who’s to say” can be separated into “who is to say.” This poses a question and the appropriate response to such a question would involve answering the question of “who.” In the above example on global warming, you could refer to David Suzuki or Al Gore.
In most instances, you can replace “who’s to say” with “what’s to say” to get the same desired query. You are still questioning the certainty of a certain statement. This is quite different than the difference between much and many, since that is much more distinct.
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