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PREFACE xxix 

to the act and for the sake of the act. Moreover a passive 
potency is not a non-active potency, but a potency which is 
passive before being operative, namely, one that must be in- 
formed by something other than itself before being in act. A 
passive potency is a mean term in a segment of change, as for 
instance the term b in the proportion a : b :: b : c; for act and 
potency being continuous must have a term in common and 
yet a difference. From what has been said it follows that every 
body has a constitutive principle by virtue of which it is that 
which it is and performs that which it performs. This is its 
proper perfection, since each creature exists by its proper act and 
perfection. Accordingly, an actual organic body is a corporeal 
substance, the organized parts of which are its matter, pos- 
sessing powers, and the soul of which is the form, the actuality 
of the aforesaid powers. 

The soul as such is the actuality of both the passive and 
active powers of the organic body. It is a principle of cognition 
that powers are known from their acts as the potentialities of 
a substance are known from its operations. In respect to the 
soul the suppositum or subject of operation is the composite 
of body and soul, that is to say, the primary and efficient act by 
virtue of which soul and body constitute a substantial unity 
rather than an accidental association. The whole man in action 
is the subject of analysis as opposed to any restricted aspect of 
him. 

A vegetative soul consists in the reduction of the operations 
of nutrition, growth and reproduction to unity. An animal, 
being one grade removed in excellence, has a soul which is a 
unity of the vegetative operations and in addition that other 
perfection which supervenes to it, namely, the operations of 
sensation, locomotion and appetite or desire (the affective 
impulses). The soul of a man is a reduction to unity of the 
vegetative and sensitive operations and another perfection