Now observe . . . the process of determining an essential characteristic: the
rule of fundamentality. When a given group of existents has more than one
characteristic distinguishing it from other existents, man must observe the
relationships among these various characteristics and discover the one on which
all the others (or the greatest number of others) depend, i.e., the fundamental
characteristic without which the others would not be possible. This fundamental
characteristic is the essential distinguishing characteristic of the
existents involved, and the proper defining characteristic of the concept.
Metaphysically, a fundamental characteristic is that distinctive characteristic
which makes the greatest number of others possible; epistemologically, it is
the one that explains the greatest number of others.