The question asked in the title is the exact question e-mailed to me today from an undergraduate mathematics student whom I know as a highly competent student. The one sentence question was accompanied with the following demo:

At one level, this question has a philosophical bent. However, I am looking for a down-to-earth, non-authoritative answer who one may give to such a student. What would be your answer if you were faced with such a question?

Update. Before the question gets closed , I should stress that the question wasn't (isn't) for you in the sense that whether you think it is a proof or not. Of course, the answer to such a question is as clear as the daylight for MO users. The question was (is) what your constructive answer would be to such a student if you were faced with such a question.