The Complete Aristotle (eng.)

is neither for the most part, nor
always and of necessity, but merely as it chances; e.g. there might
be cold in the dogdays, but this occurs neither always and of
necessity, nor for the most part, though it might happen sometimes.
The accidental, then, is what occurs, but not always nor of
necessity, nor for the most part. Now we have said what the
accidental is, and it is obvious why there is no science of such a
thing; for all science is of that which is always or for the most
part, but the accidental is in neither of these classes.
    Evidently there are not causes and principles of the accidental,
of the same kind as there are of the essential; for if there were,
everything would be of necessity. If A is when B is, and B is when
C is, and if C exists not by chance but of necessity, that also of
which C was cause will exist of necessity, down to the last
causatum as it is called (but this was supposed to be accidental).
Therefore all things will be of necessity, and chance and the
possibility of a thing’s either occurring or not occurring are
removed entirely from the range of events. And if the cause be
supposed not to exist but to be coming to be, the same results will
follow; everything will occur of necessity. For to-morrow’s eclipse
will occur if A occurs, and A if B occurs, and B if C occurs; and
in this way if we subtract time from the limited time between now
and to-morrow we shall come sometime to the already existing
condition. Therefore since this exists, everything after this will
occur of necessity, so that all things occur of necessity.
    As to that which ‘is’ in the sense of being true or of being by
accident, the former depends on a combination in thought and is an
affection of thought (which is the reason why it is the principles,
not of that which ‘is’ in this sense, but of that which is outside
and can exist apart, that are sought); and the latter is not
necessary but indeterminate (I mean the accidental); and of such a
thing the causes are unordered and indefinite.
    Adaptation to an end is found in events that happen by nature or
as the result of thought. It is ‘luck’ when one of these events
happens by accident. For as a thing may exist, so it may be a
cause, either by its own nature or by accident. Luck is an
accidental cause at work in such events adapted to an end as are
usually effected in accordance with purpose. And so luck and
thought are concerned with the same sphere; for purpose cannot
exist without thought. The causes from which lucky results might
happen are indeterminate; and so luck is obscure to human
calculation and is a cause by accident, but in the unqualified
sense a cause of nothing. It is good or bad luck when the result is
good or evil; and prosperity or misfortune when the scale of the
results is large.
    Since nothing accidental is prior to the essential, neither are
accidental causes prior. If, then, luck or spontaneity is a cause
of the material universe, reason and nature are causes before
it.
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    div id="section127" class="section" title="9">
9
    Some things are only actually, some potentially, some
potentially and actually, what they are, viz. in one case a
particular reality, in another, characterized by a particular
quantity, or the like. There is no movement apart from things; for
change is always according to the categories of being, and there is
nothing common to these and in no one category. But each of the
categories belongs to all its subjects in either of two ways (e.g.
‘this-ness’-for one kind of it is ‘positive form’, and the other is
‘privation’; and as regards quality one kind is ‘white’ and the
other ‘black’, and as regards quantity one kind is ‘complete’ and
the other ‘incomplete’, and as regards spatial movement one is
‘upwards’ and the other ‘downwards’, or one thing is ‘light’ and
another ‘heavy’); so that there are as many kinds of movement and
change as of being. There being a distinction in each class of
things between the potential and the completely real, I call the
actuality of the potential as such, movement. That what we say is
true, is plain from the following facts. When the ‘buildable’, in
so far as it is what we mean by ‘buildable’, exists actually, it is
being built, and this is the process of building. Similarly with
learning, healing, walking, leaping, ageing, ripening. Movement
takes when the complete reality itself exists, and neither earlier
nor later. The complete