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xxxii PREFACE 

rated from the matter of the substance, but retaining the con- 
ditions of individuality. Accordingly, the sensation is not sig- 
nificant as a physiological event but as a potential sign. 

The intellect is both an active and a passive power. Of these 
the passive or possible intellect is in potentiality to all intelli- 
gible things, and the active intellect is the efficient cause of the 
transmission into actuality of the potencies of the passive in- 
tellect. It is the passive intellect which is in potentiality to all 
sensible species, but things are not actually intelligible apart 
from the act of intellection. For example, corporeal substances 
are actually sensible but only potentially intelligible, so that 
they are actually intelligible only as actually understood. Con- 
sequently, it follows that intelligible things are not the efficient 
cause of the actualization of the passive intellect. Since, as 
Aristotle asserts in the DC Anima and following him St. 
Thomas, the object of knowing and the act of knowing are one 
and the same, the intelligible in act is the intellect in act. The 
actualization of the potential intelligibility of a thing is the 
same as the actualization of the passive intellect. It should be 
understood that the actual intellect is whatever it actually 
understands. 

Although the intelligible thing in potency is different from 
the passive intellect in potency, nevertheless as actualized they 
are assimilated. The first operation of the active intellect is ab- 
straction, and abstraction is the primary reduction of the passive 
intellect from potentiality to act, or in general the reduction 
of potentially intelligible things to things understood. 

Material things, as has been said, are actually sensible but not 
actually intelligible, inasmuch as their forms are individuated 
by material ingredients. Only by abstraction are they actually 
intelligible, and it may be said that things are actually intelli- 
gible in the measure in which they are abstracted from matter. 
Abstraction is the power of the active intellect to exclude in-