The Complete Aristotle (eng.)

is always desirable for the sake of
something else.
    Or see if he has put anything that exists in two genera or more
into one of them only. For some things it is impossible to place in
a single genus, e.g. the ‘cheat’ and the ‘slanderer’: for neither
he who has the will without the capacity, nor he who has the
capacity without the will, is a slanderer or cheat, but he who has
both of them. Hence he must be put not into one genus, but into
both the aforesaid genera.
    Moreover, people sometimes in converse order render genus as
differentia, and differentia as genus, defining (e.g.) astonishment
as ‘excess of wonderment’ and conviction as ‘vehemence of
conception’. For neither ‘excess’ nor ‘vehemence’ is the genus, but
the differentia: for astonishment is usually taken to be an
‘excessive wonderment’, and conviction to be a ‘vehement
conception’, so that ‘wonderment’ and ‘conception’ are the genus,
while ‘excess’ and ‘vehemence’ are the differentia. Moreover, if
any one renders ‘excess’ and ‘vehemence’ as genera, then inanimate
things will be convinced and astonished. For ‘vehemence’ and
‘excess’ of a thing are found in a thing which is thus vehement and
in excess. If, therefore, astonishment be excess of wonderment the
astonishment will be found in the wonderment, so that ‘wonderment’
will be astonished! Likewise, also, conviction will be found in the
conception, if it be ‘vehemence of conception’, so that the
conception will be convinced. Moreover, a man who renders an answer
in this style will in consequence find himself calling vehemence
vehement and excess excessive: for there is such a thing as a
vehement conviction: if then conviction be ‘vehemence’, there would
be a ‘vehement vehemence’. Likewise, also, there is such a thing as
excessive astonishment: if then astonishment be an excess, there
would be an ‘excessive excess’. Whereas neither of these things is
generally believed, any more than that knowledge is a knower or
motion a moving thing.
    Sometimes, too, people make the bad mistake of putting an
affection into that which is affected, as its genus, e.g. those who
say that immortality is everlasting life: for immortality seems to
be a certain affection or accidental feature of life. That this
saying is true would appear clear if any one were to admit that a
man can pass from being mortal and become immortal: for no one will
assert that he takes another life, but that a certain accidental
feature or affection enters into this one as it is. So then ‘life’
is not the genus of immortality.
    Again, see if to an affection he has ascribed as genus the
object of which it is an affection, by defining (e.g.) wind as ‘air
in motion’. Rather, wind is ‘a movement of air’: for the same air
persists both when it is in motion and when it is still. Hence wind
is not ‘air’ at all: for then there would also have been wind when
the air was not in motion, seeing that the same air which formed
the wind persists. Likewise, also, in other cases of the kind.
Even, then, if we ought in this instance to admit the point that
wind is ‘air in motion’, yet we should accept a definition of the
kind, not about all those things of which the genus is not true,
but only in cases where the genus rendered is a true predicate. For
in some cases, e.g. ‘mud’ or ‘snow’, it is not generally held to be
true. For people tell you that snow is ‘frozen water’ and mud is
earth mixed with moisture’, whereas snow is not water, nor mud
earth, so that neither of the terms rendered could be the genus:
for the genus should be true of all its species. Likewise neither
is wine ‘fermented water’, as Empedocles speaks of ‘water fermented
in wood’;’ for it simply is not water at all.
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6
    Moreover, see whether the term rendered fail to be the genus of
anything at all; for then clearly it also fails to be the genus of
the species mentioned. Examine the point by seeing whether the
objects that partake of the genus fail to be specifically different
from one another, e.g. white objects: for these do not differ
specifically from one another, whereas of a genus the species are
always different, so that ‘white’ could not be the genus of
anything.
    Again, see whether he has named as genus or differentia some
feature that goes with everything: for the number of attributes
that follow everything is