Although the present book pays special attention to the
requirements of students and teachers of mathematics, it
should interest anybody concerned with the ways and
means of invention and discovery. Such interest may be
more widespread than one would assume without reflec
tion. The space devoted. by popular newspapers and
magazines to crossword puzzles and other riddles seems
to show that people spend some time in solving practical problems. Behind the desire to solve this or that
problem that confers no material advantage, there may
be a deeper curiosity,a desire to understand the ways and
means, the motives and procedures,of solution. The following pages are written somewhat concisely,
but as simply as possible, and are based on long and
serious study of methods of solution. This sort of study,
called heuristic by some writers, is not in fashion now-
adays but has along past and, perhaps,some future. Studying the methods of solving problems, we perceive
another face of mathematics. Yes, mathematics has two
faces; it is the rigorous science of Euclid but it is also
someting else. Mathematics persented in the Euclidean
way appears as a systematic, deductive science; but mathematics
in the making appears as an experimental, in
ductive sciencs. Both aspects are as old as the science of
mathematics itself. But the second aspect is new in one
respect; mathematics "in statu nascendi," in the process
of being invented, has never before been presented in
quite this manner to the student, or to the teacher him-
self, or to the general public.