, be made the subject of specific
experiment. These causes are, laws of human nature, and external
circumstances capable of exciting the human will to action. The desires
of man, and the nature of the conduct to which they prompt him, are
within the reach of our observation. We can also observe what are the
objects which excite those desires. The materials of this knowledge
every one can principally collect within himself; with reasonable
consideration of the differences, of which experience discloses to him
the existence, between himself and other people. Knowing therefore
accurately the properties of the substances concerned, we may reason
with as much certainty as in the most demonstrative parts of physics
from any assumed set of circumstances. This will be mere trifling if the
assumed circumstances bear no sort of resemblance to any real ones; but
if the assumption is correct as far as it goes, and differs from the
truth no otherwise than as a part differs from the whole, then the
conclusions which are correctly deduced from the assumption constitute
_abstract_ truth; and when completed by adding or subtracting the effect
of the non-calculated circumstances, they are true in the concrete, and
may be applied to practice.

Of this character is the science of Political Economy in the writings of
its best teachers. To render it perfect as an abstract science, the
combinations of circumstances which it assumes, in order to trace their
effects, should embody all the circumstances that are common to all
cases whatever, and likewise all the circumstances that are common to
any important class of cases. The conclusions correctly deduced from
these assumptions, would be as true in the abstract as those of
mathematics; and would be as near an approximation as abstract truth can
ever be, to truth in the concrete.

When the principles of Political Economy are to be applied to a
particular ease, then it is necessary to take into account all the
individual circumstances of that case; not only examining to which of
the sets of circumstances contemplated by the abstract science the
circumstances of the case in question correspond, but likewise what
other circumstances may exist in that case, which not being common to it
with any large and strongly-marked class of cases, have not fallen under
the cognizance of the science. These circumstances have been called
_disturbing causes_. And here only it is that an element of uncertainty
e