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INTRODUCTION.

The absolutely rigorous application of the method
is, however, in almost all cases impracticable, since, so
long as. any one group were to remain imperfect, from
want of knowledge of all the facts belonging to it, the
whole of the investigation would bave to be delayed until
the deficient facts were obtained. But the process is much.
facilitated, and the progress of the investigation promoted,
by the aid of Theory and Assumption. Each of these is
a temporary substitute for approved knowledge or for
accepted fact. By their means the mind is able to experi--mentally
 complete the groups necessary for the purpose
of combining into more complex, or separating into more
elementary forms, the subjects belonging to the investication,


But herein is occasion for scrupulous care, and a
necessity for the observance of strict rule. The substibuted,
 i. e., the suppositious or assumed facts, and the
compounded knowledge thence obtained, must be kept
listinct and separate, and not be accepted by the mind as
actual knowledge, until it has been legitimately verified.
The necessary rules and systematic application of those
rules are taught and illustrated in two works, one of
which is of a more general, and the other of a more particular
 and strictly rigorous character: namely, Bacon's
Inductive System, and the work called. . Euclid’s Elements
 of Geometry.
The unsatisfactory condition of the subjects belonging
to political economy, at the present time, may be
apprehended from the work of Professor Cairnes.
The subjects have been separated inte groups of a first