I would argue that mathematics itself does not have a subject matter. It started with counting and geometry, then became applied to physics, and slowly develop

I would argue that mathematics itself does not have a subject matter. It started with counting and geometry, then became applied to physics, and slowly developed applications to probability, then computing. It seems to me that if we can reason in a rigorous and general way about any subject, the study of that reasoning is a discipline within mathematics. Philosophy, on the other hand, seems to me to in fact have a subject matter, or rather many subjects. Philosophical conversations always center on something, be it the existence of abstract ideas or the foundation of moral choices. (If you think that having a multitude of subjects constitutes having no subject, then one might make the same charge that science has no subject since it investigates sounds as well as heat flow, as well as cancer...) What I think separates philosophy from other disciplines is not the kinds of subjects it investigates, but the kinds of answers to them that it seeks: completely precise, general, and unquestionably true.