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 sentece question was accompanied with the following demo:

![enter image description here][1]

At one level, this quesion 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?
[1]: https://i.sstatic.net/aN3Uy.gif