The Complete Aristotle (eng.)

to C, i.e. that the angle in a
semicircle is a right angle. Moreover, B is identical with (b) the
defining form of A, since it is what A’s definition signifies.
Moreover, the formal cause has already been shown to be the middle.
(c) ‘Why did the Athenians become involved in the Persian war?’
means ‘What cause originated the waging of war against the
Athenians?’ and the answer is, ‘Because they raided Sardis with the
Eretrians’, since this originated the war. Let A be war, B
unprovoked raiding, C the Athenians. Then B, unprovoked raiding, is
true of C, the Athenians, and A is true of B, since men make war on
the unjust aggressor. So A, having war waged upon them, is true of
B, the initial aggressors, and B is true of C, the Athenians, who
were the aggressors. Hence here too the cause-in this case the
efficient cause-is the middle term. (d) This is no less true where
the cause is the final cause. E.g. why does one take a walk after
supper? For the sake of one’s health. Why does a house exist? For
the preservation of one’s goods. The end in view is in the one case
health, in the other preservation. To ask the reason why one must
walk after supper is precisely to ask to what end one must do it.
Let C be walking after supper, B the non-regurgitation of food, A
health. Then let walking after supper possess the property of
preventing food from rising to the orifice of the stomach, and let
this condition be healthy; since it seems that B, the
non-regurgitation of food, is attributable to C, taking a walk, and
that A, health, is attributable to B. What, then, is the cause
through which A, the final cause, inheres in C? It is B, the
non-regurgitation of food; but B is a kind of definition of A, for
A will be explained by it. Why is B the cause of A’s belonging to
C? Because to be in a condition such as B is to be in health. The
definitions must be transposed, and then the detail will become
clearer. Incidentally, here the order of coming to be is the
reverse of what it is in proof through the efficient cause: in the
efficient order the middle term must come to be first, whereas in
the teleological order the minor, C, must first take place, and the
end in view comes last in time.
    The same thing may exist for an end and be necessitated as well.
For example, light shines through a lantern (1) because that which
consists of relatively small particles necessarily passes through
pores larger than those particles-assuming that light does issue by
penetrationand (2) for an end, namely to save us from stumbling. If
then, a thing can exist through two causes, can it come to be
through two causes-as for instance if thunder be a hiss and a roar
necessarily produced by the quenching of fire, and also designed,
as the Pythagoreans say, for a threat to terrify those that lie in
Tartarus? Indeed, there are very many such cases, mostly among the
processes and products of the natural world; for nature, in
different senses of the term ‘nature’, produces now for an end, now
by necessity.
    Necessity too is of two kinds. It may work in accordance with a
thing’s natural tendency, or by constraint and in opposition to it;
as, for instance, by necessity a stone is borne both upwards and
downwards, but not by the same necessity.
    Of the products of man’s intelligence some are never due to
chance or necessity but always to an end, as for example a house or
a statue; others, such as health or safety, may result from chance
as well.
    It is mostly in cases where the issue is indeterminate (though
only where the production does not originate in chance, and the end
is consequently good), that a result is due to an end, and this is
true alike in nature or in art. By chance, on the other hand,
nothing comes to be for an end.
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    The effect may be still coming to be, or its occurrence may be
past or future, yet the cause will be the same as when it is
actually existent-for it is the middle which is the cause-except
that if the effect actually exists the cause is actually existent,
if it is coming to be so is the cause, if its occurrence is past
the cause is past, if future the cause is future. For example, the
moon was eclipsed because the earth intervened, is becoming
eclipsed because the earth is in process of intervening, will be
eclipsed because the earth will intervene, is eclipsed because the
earth intervenes.
    To take a second example: assuming that the definition of ice is
solidified water, let C be water, A solidified, B