Units in a System

One of the central understandings in linguistics is that all languages are rule-based systems. This is true of spoken and written language, of official, scholarly language as well a dialect or slang. It's always operating as and in a system.

When we study words, the purpose is always to add to what we understand about the system. That's why we don't want to just study lists of random words that share a feature; we always want to study a word in its family, to better understand how it works in the system.