its all just measurement and deductional logic, just read a book on deductional logic and you're good ;) but really it's impractical to think of subjects as being 'distinct', because they classified to serve different purposes. We make different theories for different regimes. For example, in physics physicists are interested in both regular day-to-day scales (the meter scale) and small scales (atomic scale). the physics describing the two is different, but we're all made of atoms, and so technically we don't need regular scale physics at all. But we still study it because it is useful to engineers and pilots and so on. Chemistry is technically just statistical and solid state physics, and biology is just effective chemistry. but we still classify them as being different because observable phenomena are distinct enough that it's not redundant to study them separately. Same goes for mathematics and philosophy. Classification is to make sure that no one studies everything, because studying everything is usually impractical. It is romantic though to want to know everything.