Is 1 a prime number? No, because we define it not to be. Why do we define it not to be a prime number? That's the real question. Is 0.999... = 1? Yes, because

Is 1 a prime number? No, because we define it not to be. Why do we define it not to be a prime number? That's the real question. Is 0.999... = 1? Yes, because we define decimal numbers to behave that way. Why do we define them to behave that way? That's the real question.