350  NECESSARY  TRUTH

of  a  phenomenon  are  so  fully  and  exactly  known  that  not  only
a  phenomenon  of  this  general  character,  but  just  this  very
phenomenon,  with  exactly  these  details,  and  each  in  exactly  this
amount,  must  follow  from  those  conditions,  and  from  those  only,
then  that  phenomenon  is  fully  explained."  1  But  when  we  think
of  necessary  truth,  we  must  do  so  in  terms  of  that  reality  to  which
our  minds  seem  to  testify.  If  we  question  the  reality,  we  can
declare,  "  Fact  I  doubt :  and  law  I  doubt :  and,  therefore,  necessity
I  doubt."  But,  given  the  fact  and  law,  then  necessity  is  beyond
all  doubt.
587.  When  tracing  cause  and  effect,  we  do  not  refer  explicitly
to  all  the  qualities  of  the  objects  we  are  considering.     Every  object
has  many  qualities.     Thus  iron  has  persistence  in  time,  extension
in  space,  weight,  colour,  frangibility,  ductility,  hardness,   and   a
multitude  of  others.     It  could  not  become  hot,  it  could  not  even  be
an  object  for  us,  unless  it  had  at  least  many  of  them,  for  example,
persistence  and  extension,  yet  we  do  not  think  or  say  that  the
poker   becomes   hot   because   it   has   persistence   and   extension.
Many  pokers  are  not  hot ;  even  ice  which  cannot  become  hot  has
persistence  and  extension.     We  refer  especially  to  the  quality  in
iron,  its  temperature,  which  is  changed  by  the  fire,  and  the  quality  in
the  fire  which  is  concerned  in  the  change.     The  last  is  termed  the
cause,  the  change  in  the  first  is  termed  the  effect.
