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this way, and of the infinite in general, all the parts down to that
now present are alike intermediates; so that if there is no first
there is no cause at all.
Nor can there be an infinite process downwards, with a beginning
in the upward direction, so that water should proceed from fire, earth
from water, and so always some other kind should be produced. For
one thing comes from another in two ways-not in the sense in which
'from' means 'after' (as we say 'from the Isthmian games come the
Olympian'), but either (i) as the man comes from the boy, by the boy's
changing, or (ii) as air comes from water. By 'as the man comes from
the boy' we mean 'as that which has come to be from that which is
coming to be' or 'as that which is finished from that which is being
achieved' (for as becoming is between being and not being, so that
which is becoming is always between that which is and that which is
not; for the learner is a man of science in the making, and this is
what is meant when we say that from a learner a man of science is
being made); on the other hand, coming from another thing as water
comes from air implies the destruction of the other thing. This is why
changes of the former kind are not reversible, and the boy does not
come from the man (for it is not that which comes to be something that
comes to be as a result of coming to be, but that which exists after
the coming to be; for it is thus that the day, too, comes from the
morning-in the sense that it comes after the morning; which is the
reason why the morning cannot come from the day); but changes of the
other kind are reversible. But in both cases it is impossible that the
number of terms should be infinite. For terms of the former kind,
being intermediates, must have an end, and terms of the latter kind
change back into one another, for the destruction of either is the
generation of the other.
At the same time it is impossible that the first cause, being
eternal, should be destroyed; for since the process of becoming is not
infinite in the upward direction, that which is the first thing by
whose destruction something came to be must be non-eternal.
Further, the final cause is an end, and that sort of end which
is not for the sake of something else, but for whose sake everything
else is; so that if there is to be a last term of this sort, the
process will not be infinite; but if there is no such term, there will
be no final cause, but those who maintain the infinite series
eliminate the Good without knowing it (yet no one would try to do
anything if he were not going to come to a limit); nor would there
be reason in the world; the reasonable man, at least, always acts
for a purpose, and this is a limit; for the end is a limit.
But the essence, also, cannot be reduced to another definition
which is fuller in expression. For the original definition is always
more of a definition, and not the later one; and in a series in
which the first term has not the required character, the next has
not it either. Further, those who speak thus destroy science; for it
is not possible to have this till one comes to the unanalysable terms.
And knowledge becomes impossible; for how can one apprehend things
that are infinite in this way? For this is not like the case of the
line, to whose divisibility there is no stop, but which we cannot
think if we do not make a stop (for which reason one who is tracing
the infinitely divisible line cannot be counting the possibilities
of section), but the whole line also must be apprehended by
something in us that does not move from part to part.-Again, nothing
infinite can exist; and if it could, at least the notion of infinity
is not infinite.
But if the kinds of causes had been infinite in number, then
also knowledge would have been impossible; for we think we know,
only when we have ascertained the causes, that but that which is
infinite by addition cannot be gone through in a finite time.
3
The effect which lectures produce on a hearer depends on his
habits; for we demand the language we are accustomed to, and that
which is different from this seems not in keeping but somewhat
unintelligible and foreign because of its unwontedness. For it is
the customary that is intelligible. The force of habit is shown by the
laws, in which the legendary and childish elements prevail over our
knowledge about them, owing to habit. Thus some people do not listen
to a speaker unless he speaks mathematically, others unless he gives
instances, while others expect him to cite a poet as witness. And some
want to have everything done accurately, while others are annoyed by
accuracy, either because they cannot follow the connexion of thought
or because they regard it as pettifoggery. For accuracy has
something of this character, so that as in trade so in argument some
people think it mean. Hence one must be already trained to know how to
take each sort of argument, since it is absurd to seek at the same
time knowledge and the way of attaining knowledge; and it is not
easy to get even one of the two.
The minute accuracy of mathematics is not to be demanded in all
cases, but only in the case of things which have no matter. Hence
method is not that of natural science; for presumably the whole of
nature has matter. Hence we must inquire first what nature is: for
thus we shall also see what natural science treats of (and whether
it belongs to one science or to more to investigate the causes and the
principles of things).
Book III
1
WE must, with a view to the science which we are seeking, first
recount the subjects that should be first discussed. These include
both the other opinions that some have held on the first principles,
and any point besides these that happens to have been overlooked.
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