That is the "Root", that is, the thing that, by being called Root, becomes something else. Boundaries are actually really important in this concept, as its the bounding through name that makes "[ ]" into "not-[ ]", but [ ] is not a boundary itself, for the same reason that it cannot be given a name.
The Root that is refered to as "Kara" or [ ] is the basically the opposite of a boundary, as it is a superposition of opposites and begets all causes. Kara no Kyoukai's name could be taken as a reference of this - Garden of Sinners in english, Kara no Kyoukai (Boundary of Emptiness) in JP.
If anything, the Root as a source of "Directivity" is that from which boundaries originate instead of a boundary itself.