To find a counter-example that would make the conclusion false, we need to find an instance where a woman has been president of the United States. In this case, the counter-example would be the 40th president of the United States, who can be a woman. Since there have only been 39 presidents so far at the time of this writing, this counter-example has not yet occurred. However, if at any point in the future a woman becomes the president of the United States, it would serve as a counter-example to the conclusion that a woman cannot be president.
