Student or Learner
...Category formation runs deep in the animal kingdom. Birds building a nest have an implicit category for materials that will create a good nest, including twigs, cotton, leaves, fabric, and mud, but not, say, nails, bits of wire, melon rinds, or ...
This is an exerpt from a book titled "The Organized Mind". I'd like to ask about the phrase in bold. What does "an implicit caterogy" mean? Is this a category that is not named or even recognized clearly but still exists?
I'm not a teacher. I speak American English. I've tutored writing at the University of Southern Maine and have done a good deal of copy editing and writing, occasionally for publication.