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PREFACE xxxv 

genera and species. The nature is that which is known, whereas 
the essence is that by virtue of which it is known. 

Matter is to form as genus is to species, from which it is 
to be understood that as matter is that which is differentiated 
by formal act in the natural order, so genus is that which is 
differentiated by the intellective act in the rational order. And 
indeed it is important that the analytic remotion entailed should 
be guarded and preserved if a confusion of domains of reference 
is to be avoided. Species are explicitly differentiated by the 
active intellect assigning the proper act of the thing consti- 
tuting the species as such a kind. This it does respecting the 
sensible species actualized in the senses by the active agency of 
the thing. Specific unity expresses the formal similitude of a 
universal as an univocal sign of many as referred from a sec- 
ondary substance (analytic order of reason) to multiple pri- 
mary substances (order of independent natures) which are in- 
dividuals of a like species. Since the delimiting and hence 
limiting act of the primary substance determines it to be what 
it is, the form is in its way the cause of specific unity. Nu- 
merical unity expresses the discrete oneness of the multiple 
individuals constituted according to similar specific acts. It thus 
expresses the generality of distinction and the material multi- 
plicity of primary substances having the same secondary sub- 
stance. It should be carefully noted that numerical unity, far 
from excluding specific unity, presupposes it actualized in the 
individual in the sense that the unity or indivision of the nature 
(primary operation by virtue of which a thing is what it is), 
the principle of specific unity, is of necessity antecedent to a 
substance being numerically one. The universal is a common 
sign of many, but it is such through the similarity of the act 
by which each of the many have specific unity as a precondition 
of their oneness with respect to multiplicity. Nor does the 
universal which refers to natures have an existence over and