The Complete Aristotle (eng.)

things are called
like if the qualities they have in common are more numerous than
those in which they differ-either the qualities in general or the
prominent qualities; e.g. tin is like silver, qua white, and gold
is like fire, qua yellow and red.
    Evidently, then, ‘other’ and ‘unlike’ also have several
meanings. And the other in one sense is the opposite of the same
(so that everything is either the same as or other than everything
else). In another sense things are other unless both their matter
and their definition are one (so that you are other than your
neighbour). The other in the third sense is exemplified in the
objects of mathematics. ‘Other or the same’ can therefore be
predicated of everything with regard to everything else-but only if
the things are one and existent, for ‘other’ is not the
contradictory of ‘the same’; which is why it is not predicated of
non-existent things (while ‘not the same’ is so predicated). It is
predicated of all existing things; for everything that is existent
and one is by its very nature either one or not one with anything
else.
    The other, then, and the same are thus opposed. But difference
is not the same as otherness. For the other and that which it is
other than need not be other in some definite respect (for
everything that is existent is either other or the same), but that
which is different is different from some particular thing in some
particular respect, so that there must be something identical
whereby they differ. And this identical thing is genus or species;
for everything that differs differs either in genus or in species,
in genus if the things have not their matter in common and are not
generated out of each other (i.e. if they belong to different
figures of predication), and in species if they have the same genus
(’genus’ meaning that identical thing which is essentially
predicated of both the different things).
    Contraries are different, and contrariety is a kind of
difference. That we are right in this supposition is shown by
induction. For all of these too are seen to be different; they are
not merely other, but some are other in genus, and others are in
the same line of predication, and therefore in the same genus, and
the same in genus. We have distinguished elsewhere what sort of
things are the same or other in genus.
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4
    Since things which differ may differ from one another more or
less, there is also a greatest difference, and this I call
contrariety. That contrariety is the greatest difference is made
clear by induction. For things which differ in genus have no way to
one another, but are too far distant and are not comparable; and
for things that differ in species the extremes from which
generation takes place are the contraries, and the distance between
extremes-and therefore that between the contraries-is the
greatest.
    But surely that which is greatest in each class is complete. For
that is greatest which cannot be exceeded, and that is complete
beyond which nothing can be found. For the complete difference
marks the end of a series (just as the other things which are
called complete are so called because they have attained an end),
and beyond the end there is nothing; for in everything it is the
extreme and includes all else, and therefore there is nothing
beyond the end, and the complete needs nothing further. From this,
then, it is clear that contrariety is complete difference; and as
contraries are so called in several senses, their modes of
completeness will answer to the various modes of contrariety which
attach to the contraries.
    This being so, it is clear that one thing have more than one
contrary (for neither can there be anything more extreme than the
extreme, nor can there be more than two extremes for the one
interval), and, to put the matter generally, this is clear if
contrariety is a difference, and if difference, and therefore also
the complete difference, must be between two things.
    And the other commonly accepted definitions of contraries are
also necessarily true. For not only is (1) the complete difference
the greatest difference (for we can get no difference beyond it of
things differing either in genus or in species; for it has been
shown that there is no ‘difference’ between anything and the things
outside its genus, and among the things which differ in species the
complete difference is the greatest); but also (2) the things in
the same genus