Kaiser speaks here of the issue of missing index entries in commercial and even library-based indexes versus the personal indexing of one's own card index/zettelkasten.

Some of the problem comes down to a question of scale as well as semantics, but there's also something tied up with the levels of specificity from broad category headwords to more specific, and finally down to the level of individual ideas. Some of this can be seen in the levels of specificity within the Syntopicon though there aren't any (?, doublecheck) of links from one idea directly to another.

Note that while there may be direct links from a single idea to another, there is still infinite space by which one can interpose additional ideas between them.