If we loose sight of the common-sense foundations of our subject
Everyone has common sense but that isn't the foundation for reasoned thought, or else everyone would exhibit it. If I walk into a school of English speaking students and teach them Chinese, I would begin by explaining Chinese to them, in English. But the foundation of Chinese is not English. The foundation of Chinese is the phonology, grammar and syntax of Chinese. But I have little choice in the beginning but to leverage their ability with English in the task.