by Aristotle
   Others there are who, indeed, believe that chance is a cause, but
   that it is inscrutable to human intelligence, as being a divine
   thing and full of mystery.
   Thus we must inquire what chance and spontaneity are, whether they
   are the same or different, and how they fit into our division of
   causes.
   5
   First then we observe that some things always come to pass in the
   same way, and others for the most part. It is clearly of neither of
   these that chance is said to be the cause, nor can the 'effect of
   chance' be identified with any of the things that come to pass by
   necessity and always, or for the most part. But as there is a third
   class of events besides these two-events which all say are 'by
   chance'-it is plain that there is such a thing as chance and
   spontaneity; for we know that things of this kind are due to chance
   and that things due to chance are of this kind.
   But, secondly, some events are for the sake of something, others
   not. Again, some of the former class are in accordance with deliberate
   intention, others not, but both are in the class of things which are
   for the sake of something. Hence it is clear that even among the
   things which are outside the necessary and the normal, there are
   some in connexion withwhich the phrase 'for the sake of something'
   is applicable. (Events that are for the sake of something include
   whatever may be done as a result of thought or of nature.) Things of
   this kind, then, when they come to pass incidental are said to be
   'by chance'. For just as a thing is something either in virtue of
   itself or incidentally, so may it be a cause. For instance, the
   housebuilding faculty is in virtue of itself the cause of a house,
   whereas the pale or the musical is the incidental cause. That which is
   per se cause of the effect is determinate, but the incidental cause is
   indeterminable, for the possible attributes of an individual are
   innumerable. To resume then; when a thing of this kind comes to pass
   among events which are for the sake of something, it is said to be
   spontaneous or by chance. (The distinction between the two must be
   made later-for the present it is sufficient if it is plain that both
   are in the sphere of things done for the sake of something.)
   Example: A man is engaged in collecting subscriptions for a feast.
   He would have gone to such and such a place for the purpose of getting
   the money, if he had known. He actually went there for another purpose
   and it was only incidentally that he got his money by going there; and
   this was not due to the fact that he went there as a rule or
   necessarily, nor is the end effected (getting the money) a cause
   present in himself-it belongs to the class of things that are
   intentional and the result of intelligent deliberation. It is when
   these conditions are satisfied that the man is said to have gone 'by
   chance'. If he had gone of deliberate purpose and for the sake of
   this-if he always or normally went there when he was collecting
   payments-he would not be said to have gone 'by chance'.
   It is clear then that chance is an incidental cause in the sphere of
   those actions for the sake of something which involve purpose.
   Intelligent reflection, then, and chance are in the same sphere, for
   purpose implies intelligent reflection.
   It is necessary, no doubt, that the causes of what comes to pass
   by chance be indefinite; and that is why chance is supposed to
   belong to the class of the indefinite and to be inscrutable to man,
   and why it might be thought that, in a way, nothing occurs by
   chance. For all these statements are correct, because they are well
   grounded. Things do, in a way, occur by chance, for they occur
   incidentally and chance is an incidental cause. But strictly it is not
   the cause-without qualification-of anything; for instance, a
   housebuilder is the cause of a house; incidentally, a fluteplayer
   may be so.
   And the causes of the man's coming and getting the money (when he
   did not come for the sake of that) are innumerable. He may have wished
   to see somebody or been following somebody or avoiding somebody, or
   may have gone to see a spectacle. Thus to say that chance is a thing
   contrary to rule is correct. For 'rule' applies to what is always true
   or true for the most part, whereas chance belongs to a third type of
   event. Hence, to conclude, since causes of this kind are indefinite,
   chance too is indefinite. (Yet in some cases one might raise the
   question whether any incidental fact might be the cause of the
   chance occurrence, e.g. of health the fresh air or the sun's heat
   may be the cause, but having had one's hair cut cannot; for some
   incidental causes are more relevant to the effect than others.)
   Chance or fortune is called 'good' when the result is good, 'evil'
   when it is evil. The terms 'good fortune' and 'ill fortune' are used
   when either result is of considerable magnitude. Thus one who comes
   within an ace of some great evil or great good is said to be fortunate
   or unfortunate. The mind affirms the essence of the attribute,
   ignoring the hair's breadth of difference. Further, it is with
   reason that good fortune is regarded as unstable; for chance is
   unstable, as none of the things which result from it can be invariable
   or normal.
   Both are then, as I have said, incidental causes-both chance and
   spontaneity-in the sphere of things which are capable of coming to
   pass not necessarily, nor normally, and with reference to such of
   these as might come to pass for the sake of something.
   6
   They differ in that 'spontaneity' is the wider term. Every result of
   chance is from what is spontaneous, but not everything that is from
   what is spontaneous is from chance.
   Chance and what results from chance are appropriate to agents that
   are capable of good fortune and of moral action generally. Therefore
   necessarily chance is in the sphere of moral actions. This is
   indicated by the fact that good fortune is thought to be the same,
   or nearly the same, as happiness, and happiness to be a kind of
   moral action, since it is well-doing. Hence what is not capable of
   moral action cannot do anything by chance. Thus an inanimate thing
   or a lower animal or a child cannot do anything by chance, because
   it is incapable of deliberate intention; nor can 'good fortune' or
   'ill fortune' be ascribed to them, except metaphorically, as
   Protarchus, for example, said that the stones of which altars are made
   are fortunate because they are held in honour, while their fellows are
   trodden under foot. Even these things, however, can in a way be
   affected by chance, when one who is dealing with them does something
   to them by chance, but not otherwise.
   The spontaneous on the other hand is found both in the lower animals
   and in many inanimate objects. We say, for example, that the horse
   came 'spontaneously', because, though his coming saved him, he did not
   come for the sake of safety. Again, the tripod fell 'of itself',
   because, though when it 
fell it stood on its feet so as to serve for a
   seat, it did not fall for the sake of that.
   Hence it is clear that events which (1) belong to the general
   class of things that may come to pass for the sake of something, (2)
   do not come to pass for the sake of what actually results, and (3)
   have an external cause, may be described by the phrase 'from
   spontaneity'. These 'spontaneous' events are said to be 'from
   chance' if they have the further characteristics of being the
   objects of deliberate intention and due to agents capable of that mode
   of action. This is indicated by the phrase 'in vain', which is used
   when A which is for the sake of B, does not result in B. For instance,
   taking a walk is for the sake of evacuation of the bowels; if this
   does not follow after walking, we say that we have walked 'in vain'
   and that the walking was 'vain'. This implies that what is naturally
   the means to an end is 'in vain', when it does not effect the end
   towards which it was the natural means-for it would be absurd for a
   man to say that he had bathed in vain because the sun was not
   eclipsed, since the one was not done with a view to the other. Thus
   the spontaneous is even according to its derivation the case in
   which the thing itself happens in vain. The stone that struck the
   man did not fall for the purpose of striking him; therefore it fell
   spontaneously, because it might have fallen by the action of an
   agent and for the purpose of striking. The difference between
   spontaneity and what results by chance is greatest in things that come
   to be by nature; for when anything comes to be contrary to nature,
   we do not say that it came to be by chance, but by spontaneity. Yet
   strictly this too is different from the spontaneous proper; for the
   cause of the latter is external, that of the former internal.
   We have now explained what chance is and what spontaneity is, and in
   what they differ from each other. Both belong to the mode of causation
   'source of change', for either some natural or some intelligent
   agent is always the cause; but in this sort of causation the number of
   possible causes is infinite.
   Spontaneity and chance are causes of effects which though they might
   result from intelligence or nature, have in fact been caused by
   something incidentally. Now since nothing which is incidental is prior
   to what is per se, it is clear that no incidental cause can be prior
   to a cause per se. Spontaneity and chance, therefore, are posterior to
   intelligence and nature. Hence, however true it may be that the
   heavens are due to spontaneity, it will still be true that
   intelligence and nature will be prior causes of this All and of many
   things in it besides.
   7
   It is clear then that there are causes, and that the number of
   them is what we have stated. The number is the same as that of the
   things comprehended under the question 'why'. The 'why' is referred
   ultimately either (1), in things which do not involve motion, e.g.
   in mathematics, to the 'what' (to the definition of 'straight line' or
   'commensurable', c.), or (2) to what initiated a motion, e.g. 'why
   did they go to war?-because there had been a raid'; or (3) we are
   inquiring 'for the sake of what?'-'that they may rule'; or (4), in the
   case of things that come into being, we are looking for the matter.
   The causes, therefore, are these and so many in number.
   Now, the causes being four, it is the business of the physicist to
   know about them all, and if he refers his problems back to all of
   them, he will assign the 'why' in the way proper to his science-the
   matter, the form, the mover, 'that for the sake of which'. The last
   three often coincide; for the 'what' and 'that for the sake of
   which' are one, while the primary source of motion is the same in
   species as these (for man generates man), and so too, in general,
   are all things which cause movement by being themselves moved; and
   such as are not of this kind are no longer inside the province of
   physics, for they cause motion not by possessing motion or a source of
   motion in themselves, but being themselves incapable of motion.
   Hence there are three branches of study, one of things which are
   incapable of motion, the second of things in motion, but
   indestructible, the third of destructible things.
   The question 'why', then, is answered by reference to the matter, to
   the form, and to the primary moving cause. For in respect of coming to
   be it is mostly in this last way that causes are investigated-'what
   comes to be after what? what was the primary agent or patient?' and so
   at each step of the series.
   Now the principles which cause motion in a physical way are two,
   of which one is not physical, as it has no principle of motion in
   itself. Of this kind is whatever causes movement, not being itself
   moved, such as (1) that which is completely unchangeable, the
   primary reality, and (2) the essence of that which is coming to be,
   i.e. the form; for this is the end or 'that for the sake of which'.
   Hence since nature is for the sake of something, we must know this
   cause also. We must explain the 'why' in all the senses of the term,
   namely, (1) that from this that will necessarily result ('from this'
   either without qualification or in most cases); (2) that 'this must be
   so if that is to be so' (as the conclusion presupposes the premisses);
   (3) that this was the essence of the thing; and (4) because it is
   better thus (not without qualification, but with reference to the
   essential nature in each case).
   8
   We must explain then (1) that Nature belongs to the class of
   causes which act for the sake of something; (2) about the necessary
   and its place in physical problems, for all writers ascribe things
   to this cause, arguing that since the hot and the cold, c., are of
   such and such a kind, therefore certain things necessarily are and
   come to be-and if they mention any other cause (one his 'friendship
   and strife', another his 'mind'), it is only to touch on it, and
   then good-bye to it.
   A difficulty presents itself: why should not nature work, not for
   the sake of something, nor because it is better so, but just as the
   sky rains, not in order to make the corn grow, but of necessity?
   What is drawn up must cool, and what has been cooled must become water
   and descend, the result of this being that the corn grows. Similarly
   if a man's crop is spoiled on the threshing-floor, the rain did not
   fall for the sake of this-in order that the crop might be
   spoiled-but that result just followed. Why then should it not be the
   same with the parts in nature, e.g. that our teeth should come up of
   necessity-the front teeth sharp, fitted for tearing, the molars
   broad and useful for grinding down the food-since they did not arise
   for this end, but it was merely a coincident result; and so with all
   other parts in which we suppose that there is purpose? Wherever then
   all the parts came about just what they would have been if they had
   come be for an end, such things survived, b
eing organized
   spontaneously in a fitting way; whereas those which grew otherwise
   perished and continue to perish, as Empedocles says his 'man-faced
   ox-progeny' did.
   Such are the arguments (and others of the kind) which may cause
   difficulty on this point. Yet it is impossible that this should be the
   true view. For teeth and all other natural things either invariably or
   normally come about in a given way; but of not one of the results of
   chance or spontaneity is this true. We do not ascribe to chance or
   mere coincidence the frequency of rain in winter, but frequent rain in
   summer we do; nor heat in the dog-days, but only if we have it in
   winter. If then, it is agreed that things are either the result of
   coincidence or for an end, and these cannot be the result of
   coincidence or spontaneity, it follows that they must be for an end;
   and that such things are all due to nature even the champions of the
   theory which is before us would agree. Therefore action for an end
   is present in things which come to be and are by nature.
   Further, where a series has a completion, all the preceding steps
   are for the sake of that. Now surely as in intelligent action, so in
   nature; and as in nature, so it is in each action, if nothing
   interferes. Now intelligent action is for the sake of an end;
   therefore the nature of things also is so. Thus if a house, e.g. had
   been a thing made by nature, it would have been made in the same way
   as it is now by art; and if things made by nature were made also by
   art, they would come to be in the same way as by nature. Each step
   then in the series is for the sake of the next; and generally art
   partly completes what nature cannot bring to a finish, and partly
   imitates her. If, therefore, artificial products are for the sake of
   an end, so clearly also are natural products. The relation of the
   later to the earlier terms of the series is the same in both. This
   is most obvious in the animals other than man: they make things
   neither by art nor after inquiry or deliberation. Wherefore people