Your ruler will probably not help you to formulate an argument either for or against the thesis of mathematical realism. The SEP article begins by noting Frege's simple argument:
Platonism about mathematics (or mathematical platonism) is the metaphysical view that there are abstract mathematical objects whose existence is independent of us and our language, thought, and practices. Just as electrons and planets exist independently of us, so do numbers and sets. And just as statements about electrons and planets are made true or false by the objects with which they are concerned and these objects’ perfectly objective properties, so are statements about numbers and sets. Mathematical truths are therefore discovered, not invented.
The most important argument for the existence of abstract mathematical objects derives from Gottlob Frege and goes as follows (Frege 1953). The language of mathematics purports to refer to and quantify over abstract mathematical objects. And a great number of mathematical theorems are true. But a sentence cannot be true unless its sub-expressions succeed in doing what they purport to do. So there exist abstract mathematical objects that these expressions refer to and quantify over.
Platonism in the Philosophy of Mathematics (Stanford Encyclopedia of Philosophy)
I stole Frege's argument and simplified it even further (because one can never get too simplified for RF). I also mentioned, and the SEP article article cites and links to its article on, the Quine-Putnam indispensability argument:
Indispensability Arguments in the Philosophy of Mathematics (Stanford Encyclopedia of Philosophy)
I find the indispensability argument compelling, as I would think that anyone who concurs with the thesis of scientific realism should. And how can one not concur with the thesis of scientific realism? The astounding predictive abilities of modern mathematical-scientific theories, namely those of physics, would seem to me to be inexplicable any other way than by the thesis of scientific realism. No?
Anyway, I provide the above articles because, if I remember correctly, you have had difficulty finding truly intelligent philosophers to read. There are a hell of a lot of references to intelligent philosophers in the above articles. If you haven't read the works of these philosophers, I definitely recommend them over the (admittedly more numerous) idiotic philosophers.