(5) The Cause is the Unconditional Antecedent.  A cause is never simple, but may be analysed into several conditions; and ‘Condition’ means any necessary factor of a Cause:  any thing or agent that exerts, absorbs, transforms, or deflects energy; or any relation of time or space in which agents stand to one another.  A positive condition is one that cannot be omitted without frustrating the effect; a negative condition is one that cannot be introduced without frustrating the effect.  In the falling of the picture, e.g., the positive conditions were the picture (as being heavy), the slamming of the door, and the weakness of the cord:  a negative condition was that the picture should have no support but the cord.  When Mill, then, defines the Cause of any event as its “unconditional” antecedent, he means that it is that group of conditions (state and process of things) which, without any further condition, is followed by the event in question:  it is the least antecedent that suffices, positive conditions being present and negative absent.

Whatever item of the antecedent can be left out, then, without affecting the event, is no part of the cause.  Earthquakes have happened in New Zealand and votes of censure in the House of Commons without a picture’s falling in this room:  they were not unconditional antecedents; something else was needed to bring down a picture.  Unconditionality also distinguishes a true cause from an invariable antecedent that is only a co-effect:  for when day follows night something else happens; the Earth rotates upon her axis:  a flash of gunpowder is not an unconditional antecedent of a report; the powder must be ignited in a closed chamber.

By common experience, and more precisely by experiment, it is found possible to select from among the antecedents of an event a certain number upon which, so far as can be perceived, it is dependent, and to neglect the rest:  to purge the cause of all irrelevant antecedents is the great art of inductive method.  Remote or minute conditions may indeed modify the event in ways so refined as to escape our notice.  Subject to the limitations of our human faculties, however, we are able in many cases to secure an unconditional antecedent upon which a certain event invariably follows.  Everybody takes this for granted:  if the gas will not burn, or a gun will not go off, we wonder ’what can be wrong with it,’ that is, what positive condition is wanting, or what negative one is present.  No one now supposes that gunnery depends upon those “remotest of all causes,” the stars, or upon the sun being in Sagittarius rather than in Aquarius, or that one shoots straightest with a silver bullet, or after saying the alphabet backwards.