Fourth Point

Suddenly with the introduction of a fourth point, we have an entirely new situation. We can put that fourth point anywhere we choose, except in the same plane as the first three, and we invariably divide space into two sections: that which is Inside the four-point system and that which is Outside. Unwittingly, we have created the minimum System. (Similarly, mathematics requires exactly four noncoplanar points to define a Sphere.)

A System, says Bucky, is a "conceivable entity" dividing Universe into two parts: the Inside and the Outside of the system. That's it (except, of course, for the part of Universe doing the dividing; he demands precision).