Perhaps I am missing the point of reference, but this phrase might be about something like:
If you’ve never seen an alien city, it might be difficult to describe it in advance. (reference to unknown parts of reality)
Things which cannot be referenced “directly”* - perhaps by existence. The members of the class of “colors we can see” for instance is one we know exists (has members, which we can point to). But colors we can’t see may be a recently established phenomenon. While it could be “referenced”, the reference didn’t always point to a phenomenon with known examples—in fact its defining trait was that (via the known means at the time) it wasn’t (easily, known to be) observable.
How can there be an unreferenceable entity?
Perhaps I am missing the point of reference, but this phrase might be about something like:
If you’ve never seen an alien city, it might be difficult to describe it in advance. (reference to unknown parts of reality)
Things which cannot be referenced “directly”* - perhaps by existence. The members of the class of “colors we can see” for instance is one we know exists (has members, which we can point to). But colors we can’t see may be a recently established phenomenon. While it could be “referenced”, the reference didn’t always point to a phenomenon with known examples—in fact its defining trait was that (via the known means at the time) it wasn’t (easily, known to be) observable.
*Yet.