Narrowing a topic - no matter how complex it is, or what field it's in, or where you begin, or what you're doing - is really a matter of combining concepts. This holds true whether you are trying to gather information (searching) or trying to think through your research topic for a paper.
Essentially, adding concepts together creates narrower concepts. this is iterable and cumulative, and the more concepts you add, the narrower the field of inquiry becomes. To put this in visual terms, we can refer to a familiar Venn diagram illustration.
