This  is  the  force  of  the  earth's  attraction,  or  '  g/

This  detailed  explanation  has  been  necessary,
because  the  conceptions  involved  are  essential  to
an  understanding  of  the  theory  of  Ostwald,  as  well
as  of  much  of  the  contents  of  this  book.  There  are
here  four  fundamental  ideas  —  energy,  mass,  time,
and  space.  If  any  three  of  them  are  known,  the
fourth  can  be  deduced  by  the  rule  of  three.  Now,


TWO  VIEWS  CONCERNING  NATURE

from  the  beginning  of  the  world,  the  three  funda-
mentals chosen  have  been  mass,  time,  and  space  or
distance  ;  and  energy,  a  modern  conception,  has
been  derived  from  these.  But  it  is  equally  possible
to  assume  any  other  three  of  the  four  conceptions
as  fundamental,  and  to  deduce  the  fourth.  Ostwald
proposed  to  assume  energy  as  a  fundamental
concept,  or,  as  the  Germans  would  say,  a  "  Ding
an  sich,"  a  thing  by  itself,  instead  of  mass  ;  in
that  case  mass,  M— 2E/C2.
