§  68.  In  correspondence  with  the  formula,  sup-
pose a  south  wind,  Auster,  from  over  a  marsh,  to  be
attended  by  ague  in  three  several  instances.  In  the
first,  the  weather  is  Bleak  and  blighting,  also  Cloudy
and  cold,  but  not  Damp  or  Foul.  In  the  second,  it
is  Bleak  and  blighting.  Damp  and  dewy,  but  not
Cloudy  or  Foul.  In  the  third,  it  is  Cloudy  and  cold,
Foul  and  foggy,  but  not  Bleak  or  Damp.  The  meth-
od of  agreement  concludes  from  these  cases  agree-
ing in  presence  that  probably  the  ague  was  in  each
caused  by  Auster  charged  with  malaria  from  the
marsh.
Again,  suppose  in  the  same  locality  another  trio  of
winds  not  Austral  and  not  attended  by  ague,  but
each  of  the  other  circumstances  appearing  in  turn  in
one  or  two  instances,  yet  no  one  in  all  three.  The
method  of  agreement  infers  negatively  from  these
negative  cases  agreeing  in  absence  that  these  winds
not  Austral  did  not  cause  ague.
Now  this  negative  inference  greatly  strengthens
the  prior  conclusion  that  in  those  cases  the  ague  was
caused  by  the  malarial  Auster.  For,  imagine  a  se-
ries of  negative  cases  exhaustive  of  the  important
circumstances  associated  in  any  instance  with  A,  a.
This  series  alone  would  furnish  full  proof  of  their
causal  connection,  as  follows :  Generalizing  from  a
colligation  of  the  negative  cases,  we  have —


128  ELEMENTS   OF   INDUCTIVE   LOGIC

If  A  is  not,  then  a  is  not ;  and  v. v. ;
But  in  a  certain  case  a  is ;
.*.  In  that  particular  case  ^  is  ;  or  v.  v. — Tollens.
