is derived the doctrine of fate insisted on by the Stoics.  And as I have thus divided the genera of causes, without which nothing can be effected, so also the genera of the efficient causes can be divided in the same manner.  For there are some causes which manifestly produce the effect, without any assistance from any quarter; others which require external aid; as for instance, wisdom alone by herself makes men wise; but whether she is able alone to make men happy is a question.

XVI.  Wherefore, when any cause efficient as to some particular end has inevitably presented itself in a discussion, it is allowable without any hesitation to conclude that what that cause must inevitably effect is effected.  But when the cause is of such a nature that it does not inevitably effect the result, then the conclusion which follows is not inevitable And that description of causes which has an inevitable effect does not usually engender mistakes; but this description, without which a thing cannot take place, does often cause perplexity.  For it does not follow, because sons cannot exist without parents, that there was therefore any unavoidable cause in the parents to have children.  This, therefore, without which an effect cannot be produced, must be carefully separated from that by which it is certainly produced.  For that is like—­

  “Would that the lofty pine on Pelion’s brow
  Had never fall’n beneath the woodman’s axe!”

For if the beam of fir had never fallen to the ground, that Argo would not have been built; and yet there was not in the beams any unavoidably efficient power.  But when

“The fork’d and fiery bolt of Jove”

was hurled at Ajax’s vessel, that ship was then inevitably burnt.

And again, there is a difference between causes, because some are such that without any particular eagerness of mind, without any expressed desire or opinion, they effect what is, as it were, their own work; as for instance, “that everything must die which has been born.”  But other results are effected either by some desire or agitation of mind, or by habit, or nature, or art, or chance.  By desire, as in your case, when you read this book; by agitation, as in the case of any one who fears the ultimate issue of the present crisis; by habit, as in the case of a man who gets easily and rapidly in a passion; by nature, as vice increases every day; by art, as in the case of a man who paints well; by chance, as in the case of a man who has a prosperous voyage.  None of these things are without some cause, and yet none of them are wholly owing to any single cause.  But causes of this kind are not necessary ones.