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36 CONCERNING BEING AND ESSENCE
for instance risibility in man ensues from the form, since
a laugh arises from some apprehension in the soul of a
man.
One should know, too, that accidents are sometimes caused
by the essential principles according to perfect act, as for in-
stance heat in fire which is always hot in act; but at times
(they are caused) only according to an aptitude, with comple-
tion accruing to them from an exterior agent, as for instance
transparency in the air which is completed by means of a lucid
external body. And in such instances the aptitude is an in-
separable accident, but the complement, which ensues to it
from some principle which is outside the essence of the thing
or which does not enter into its constitution, will be separable,
as for instance to be moved and things of this sort,
One should know therefore that in accidents genus, species
and difference are taken in a mode other than that in which
they are taken in substances. For since in substances there is
effected from the substantial form and the matter a unity by
virtue of itself {per se unum: a substantial unity), a certain
nature resulting from their conjunction which is properly
placed in the predicament (category) of substance, therefore in
substances the concrete names which signify the composite are
properly said to be in a genus, whether species or genus, as
man or animal. However, neither form nor matter is in a
predicament (category) in this mode except through reduction,
as the former is said to be in a genus. But a substantial unity
(unum per se) is not effected from an accident and its subject,
and therefore no nature results from their conjunction to which
the intention of genus or species can be attributed. Accordingly,
the accidental names expressing a concretion, as for instance
white man or musician, are not placed in a predicament (cate-
gory), either as species or as genus, except by reduction, for
they can be placed in a predicament only according to what is