Is using "I don't know that" instead of "I don't know if" grammatically correct?

Question

When asked whether he recognized equal rights of atheists, George H. W. Bush said¹:> No, I don't know that atheists should be considered as citizens, nor> should they be considered patriots. This is one nation under God.I would understand if he had used another verb:> No, I don't think/believe that atheists should be considered...I would also understand if he had chosen another conjunction:>No, I don't know if atheists should be considered...Is the original quoted sentence grammatically correct? Is it common to use **know** in the sense of **think**? Or is this some dialect-specific meaning? Could you please give some more examples of this usage of **know**?^{1 [Wikipedia source]( http://en.wikipedia.org/wiki/Discrimination_against_atheists#United_States)}

Answer 1

The correct phrasing would be "I don't know whether . . . ". If should only be used as a conditional that may or may not occur. Never use "I don't know that" unless "that" is an indefinite article. For example: What is the capital of China? I'm sorry, I don't know that".

Please do not look to the speeches of the 43rd president for correct grammar.