The Complete Aristotle (eng.)

derived from the
extremes signs, that derived from the middle term the index: for
that which is proved through the first figure is most generally
accepted and most true.
    It is possible to infer character from features, if it is
granted that the body and the soul are changed together by the
natural affections: I say ‘natural’, for though perhaps by learning
music a man has made some change in his soul, this is not one of
those affections which are natural to us; rather I refer to
passions and desires when I speak of natural emotions. If then this
were granted and also that for each change there is a corresponding
sign, and we could state the affection and sign proper to each kind
of animal, we shall be able to infer character from features. For
if there is an affection which belongs properly to an individual
kind, e.g. courage to lions, it is necessary that there should be a
sign of it: for ex hypothesi body and soul are affected together.
Suppose this sign is the possession of large extremities: this may
belong to other kinds also though not universally. For the sign is
proper in the sense stated, because the affection is proper to the
whole kind, though not proper to it alone, according to our usual
manner of speaking. The same thing then will be found in another
kind, and man may be brave, and some other kinds of animal as well.
They will then have the sign: for ex hypothesi there is one sign
corresponding to each affection. If then this is so, and we can
collect signs of this sort in these animals which have only one
affection proper to them-but each affection has its sign, since it
is necessary that it should have a single sign-we shall then be
able to infer character from features. But if the kind as a whole
has two properties, e.g. if the lion is both brave and generous,
how shall we know which of the signs which are its proper
concomitants is the sign of a particular affection? Perhaps if both
belong to some other kind though not to the whole of it, and if, in
those kinds in which each is found though not in the whole of their
members, some members possess one of the affections and not the
other: e.g. if a man is brave but not generous, but possesses, of
the two signs, large extremities, it is clear that this is the sign
of courage in the lion also. To judge character from features,
then, is possible in the first figure if the middle term is
convertible with the first extreme, but is wider than the third
term and not convertible with it: e.g. let A stand for courage, B
for large extremities, and C for lion. B then belongs to everything
to which C belongs, but also to others. But A belongs to everything
to which B belongs, and to nothing besides, but is convertible with
B: otherwise, there would not be a single sign correlative with
each affection.

Posterior Analytics, Book I
    Translated by G. R. G. Mure
1
    All instruction given or received by way of argument proceeds
from pre-existent knowledge. This becomes evident upon a survey of
all the species of such instruction. The mathematical sciences and
all other speculative disciplines are acquired in this way, and so
are the two forms of dialectical reasoning, syllogistic and
inductive; for each of these latter make use of old knowledge to
impart new, the syllogism assuming an audience that accepts its
premisses, induction exhibiting the universal as implicit in the
clearly known particular. Again, the persuasion exerted by
rhetorical arguments is in principle the same, since they use
either example, a kind of induction, or enthymeme, a form of
syllogism.
    The pre-existent knowledge required is of two kinds. In some
cases admission of the fact must be assumed, in others
comprehension of the meaning of the term used, and sometimes both
assumptions are essential. Thus, we assume that every predicate can
be either truly affirmed or truly denied of any subject, and that
‘triangle’ means so and so; as regards ‘unit’ we have to make the
double assumption of the meaning of the word and the existence of
the thing. The reason is that these several objects are not equally
obvious to us. Recognition of a truth may in some cases contain as
factors both previous knowledge and also knowledge acquired
simultaneously with that recognition-knowledge, this latter, of the
particulars actually falling under the universal and therein
already virtually known. For example, the student knew beforehand
that the angles of every triangle are equal to two right angles;
but it